The Best Freshwater Fish For Your Aquarium

Setting up a freshwater aquarium can be an exciting venture for those who are passionate about fishkeeping. Plus, opting for this type of aquarium comes with plenty of variety, as there are many different species to choose from. 

In this article, we answer the most frequently asked questions surrounding freshwater tanks and provide recommendations on the best freshwater fish to choose when setting up one of your own. 

Plus, we’ve included plenty of care and maintenance tips which cover how to keep your aquarium and fish healthy, so stick around to find out everything you need to know. 

Read on to discover more about freshwater fish…

Two fish swimming together

What are freshwater fish?

Freshwater fish are fish species which spend some or all their life in freshwater. Typical freshwater environments include streams, rivers, ponds, and lakes. Freshwater fish are found worldwide, and make up around 41% of all known fish species.  

Freshwater fish are specially adapted to life in freshwater. Their gills have evolved to diffuse dissolved gases (such as oxygen) while keeping salts in their body. They also have specially adapted kidneys, which reclaim salts from body fluids before excretion. 

Due to the abundance of freshwater fish options available, there are plenty of choices suitable for beginners as well as more unique and exotic species that would be much more suited to experienced aquarists. 

Some freshwater fish, such as goldfish, can tolerate a wide variety of conditions and are therefore a great option for beginners. Other fish species, such as discus, require more specific water conditions and so are generally kept by experienced aquarists. 

Regardless of what fish you’re planning on keeping, it is best to speak with your local shop so you can create optimal water conditions for your new pets.

Are tropical fish freshwater or saltwater?

Tropical fish do NOT live in saltwater. Tropical fish are freshwater fish which are endemic to the tropics and thus require warmer water temperatures – typically 23-28°C. In aquariums, this is usually achieved by installing a fish tank heater in conjunction with an aquarium thermometer. Fish which live in lower temperatures (below 23°C) are known as temperate fish.

Tropical fish are the most popular group of fish to keep in tanks and aquariums. This is due to them generally being low-cost and easy to keep. Additionally, there is an incredible amount of choice, and many varieties are very colourful in appearance. Of course, this is a generalisation, and certain wild-caught or rare species may be incredibly expensive and difficult to care for. 

Tropical fish in shops are generally either wild-caught or bred in captivity. In instances where fish are bred in captivity, there are often many body morphs and colour variants, such as elongated fins and albinos.

Tropical fish swimming together in an aquarium

Can freshwater fish live in saltwater?

Most freshwater fish cannot live in true saltwater, however, some species may be able to tolerate low levels of salt. For example, tonic salt is often used in freshwater aquariums as a mild antiseptic. 

Saltwater is defined as water with a salinity of 1.020 – 1.028 specific gravity (sg). There are some fish which can tolerate moving between the two, and this is usually for reproductive purposes. We’ve included examples of two fish species below that are capable of surviving in both freshwater and saltwater environments. 

Anadromous fish

Anadromous fish are born in fresh water and migrate to saltwater to spend most of their adult life. They usually then return to freshwater to spawn. 

The most well-known anadromous fish is the salmon. Catadromous fish do the exact opposite. Catadromous fish are born in the sea and migrate to freshwater systems to grow and spend the majority of their adult life. They then return to the sea to spawn. Examples of catadromous fish include the European eel. 


Brackish fish 

Brackish fish are fish that can tolerate a level of salt between that of saltwater and freshwater (e.g., 1.015 specific gravity). Brackish water, which is a mixture of freshwater and saltwater, is commonly found in estuaries. An example of this would be where a river meets the sea.

Brackish fish will often tolerate freshwater and marine water, but will not thrive or survive in either for long periods. Typical examples of brackish water fish include archerfish and the Green-Spotted puffer.  

Archer fish swimming

How many freshwater fish species are there in the UK?

There are approximately 42 species of native, freshwater fish species in the UK.

Typical examples of freshwater fish species in the UK include carp, roach, rudd, tench, perch, trout, pike and grayling. Additionally, there are several anadromous/catadromous species which spend some time in our freshwater systems when they’re not migrating to or from the sea. These include salmon, sea-trout, European eels and lamprey eels. 

Top tips on how to keep freshwater fish

Keeping fish is all down to your water. Remember, if we keep our water right, then our fish should do well. Aquarium hobbyists are often referred to as water keepers rather than fish keepers because of this. 

Water quality is a general term which relates to the water being acceptable for the fish. Of course, different fish have different requirements, therefore, it is vital you do your research before buying fish. 

When we talk about water quality, we’re referring to the water being free of potentially harmful compounds, such as ammonia and nitrite, as well as having the correct pH and water hardness (i.e. the level of dissolved minerals in the water).


Ammonia (NH3) is a chemical compound which is created in aquariums from decaying organic matter, such as fish waste. Ammonia can poison aquarium fish when it is present at high levels, so we must reduce levels to a minimum. Unfortunately, ammonia is an essential part of maturing a fish tank filter. Ammonia is broken down by a specialist bacteria (known as Nitrosomonas), which converts the product into nitrites. 

To grow these essential bacteria colonies, we need a level of ammonia to feed them. The most common way of feeding these colonies is by adding a small number of fish to begin the nitrogen cycle. Alternatively, we can also add liquid ammonia to the aquarium to simulate the addition of fish. It takes time for the bacteria colonies to grow, but we would recommend adding liquid bacteria to the aquarium to aid the development of these colonies and speed the process up. 

Ammonia will be present in all new aquariums, but it can also be present in fish tanks which are experiencing problems. There are several reasons why ammonia levels may spike in established aquariums: 

> Over-feeding -  if the fish are producing more waste than the filter and bacteria colonies can handle, then this will cause a potentially problematic ammonia spike. 

Maintenance & cleaning - during an aquarium clean, we use a siphon to remove organic matter from the substrate. If we don’t do this often enough, there will be a build-up of decaying organic matter which can again lead to an ammonia spike. It is best practice to clean your aquarium weekly with a gravel cleaner.

> Filter cleaning - filters consist of porous materials (such as sponges) in which bacterial colonies can comfortably live in. If we wash our filter media with chlorinated tap water, it’ll unfortunately damage or kill the bacteria colonies which process ammonia. This is like pushing the reset button in an aquarium, which can ultimately lead to an ammonia spike. Chlorine in our tap water is designed to kill potentially harmful bacteria. This is great for us, but it also kills good bacteria which makes it not so ideal for fish tanks.

> Too many fish – if our aquarium is overstocked (and under-filtered), there will simply be too much waste produced in an aquarium for the filters to handle. In these instances, it is best to re-home fish or upgrade your fish tank filter

> Acidic water – acidic water can kill bacteria colonies. Ensure your water pH is correct by adding aquarium buffer to your aquarium.

Buy your freshwater ammonia test here…


As mentioned in the above paragraphs, Nitrosomonas break down poisonous ammonia (NH3) into nitrites (NO2). Nitrites are less toxic to fish, however, they still have the potential to cause problems. If they’re present in your water tests, it means the Nitrosomonas colonies are growing and doing are doing their job by converting the ammonia. 

To convert harmful nitrites into nitrates (NO3), we must grow another bacteria in our filters known as Nitrobacter, which feeds on nitrites. Nitrobacter colonies will only begin to grow once there are sufficient levels of nitrites in the aquarium, which can take several weeks in new fish tanks. You will find that the ammonia levels will be the first to diminish, with the nitrite levels falling soon after. 

As we mentioned above, nitrites are common in new tanks going through the nitrogen cycle. If you have nitrites in an established aquarium, this is usually the result of an issue which is being resolved. 

Bacteria colonies need oxygen to thrive, which is extracted from water flowing through the aquarium filters. If your filters are clogged and water cannot pass through the media, then bacteria colonies will suffer - which is one of the many reasons why filter maintenance is so important! So, for the best results, clean lean your filter media in old aquarium water.

Buy your freshwater nitrite test here…


Nitrates (NO3) are the final compound of the nitrogen cycle. Nitrates are not harmful to fish, but they’re a resource for aquatic plants, particularly algae. High nitrates can often result in algae blooms, which can make aquariums look untidy. Nitrates will gas off fish tanks to an extent, but they are best removed via water changes. If we replace 10-20% of the water in our aquarium with fresh dechlorinated water every week, we can lower the nitrate levels to an acceptable range, which will improve the overall water clarity.

Buy your freshwater nitrate test here…


pH is a scale used in chemistry to determine how acidic or alkaline a liquid is. In this case, we are looking at the pH of our aquarium water, and having a consistent pH is key to having a thriving aquarium. 

As a rule of thumb, the acceptable pH of aquarium water ranges from pH 6.8 – 7.8, with pH 7.0 being the neutral point. However, please note that pH can be species-specific and certain fish types  (e.g. South American cichlids) may prefer much more acidic water, which can be harmful to most fish.

Nitric acid is a by-product of the nitrogen cycle. In soft water (i.e. water lacking minerals), the nitric acid in aquarium water can quickly lead to acidic conditions (e.g. pH 6) which can damage fish and upset bacteria colonies in filters.

This can create problems such as ammonia/nitrite spikes, which can be fatal to fish. Likewise, areas with extremely hard water (i.e. water with high mineral content) may experience alkaline conditions (e.g. pH 8) which some species (e.g. cardinal tetras) will not like. It is therefore important to understand the requirements of your inhabitants and test your water pH regularly. 

Buy your freshwater pH test here…

Water hardness

Water hardness is the measurement of dissolved magnesium and calcium in the water. It is measured on a scale from 0-17, with the higher numbers indicating a larger dissolved mineral content. There are two types of water hardness – KH and GH. 

KH (also known as carbonate hardness) is the degree to which a fish tank can buffer against pH fluctuations. By having a suitable KH, we can be sure our pH remains consistent - which is key to happy aquariums. Most aquariums benefit from a KH of 3, which will ensure our pH remains at pH 7 or higher. It’s worth mentioning that fish and aquarium plants are sensitive to pH swings, so monitoring this is incredibly important! 

GH (also known as general hardness) measures the amount of dissolved magnesium and calcium in the water. GH is important for muscle development, and organ function of most fish. Likewise, freshwater shrimp are very sensitive to GH and prefer hard water. 

Aquatic plants will also utilise magnesium and calcium in their development. A suitable GH level for most aquariums would be 3, however, some species (e.g. Malawi cichlids) prefer much higher levels of GH at around 10-17. 

Our tap water determines our water hardness, so we must test and understand this to aid our decisions before adjusting. If you need to purchase GH and KH minerals, you can do so conveniently online from Complete Koi and Aquatics. Alternatively, if you would like more information on this topic, click here to read our complete guide to understanding water hardness.

Buy your freshwater hardness test here…


All fish species have a temperature range. Likewise, the bacteria colonies living in the filters will do best when the temperatures are consistent. It is therefore important to monitor fish tank temperatures with a thermometer, while consistency can be achieved using an aquarium heater set to the correct level. 

If you are keeping tropical fish, your aquarium heater will need to be set between 24-27°C. If you’re keeping temperate fish species, your aquarium heater will need to be set between 18-22°C. 

When it comes to buying an aquarium heater, you will require 1 watt (W) of power per 1 litre (L) of water. So, for example, if you have a 100L aquarium, you would be best purchasing a 100W heater. 

We recommend always keeping an eye out for signs that your heater is breaking, as they are the most common pieces of kit to fail! And a sudden temperature drop can be fatal to aquarium fish - so it’s always a good idea to regularly check on your heater equipment.

Keep aquariums clean

Aquarium maintenance is the cornerstone of all aquariums. Failure to maintain aquariums correctly will usually result in fatalities, as well as an unsightly foul-smelling aquarium. The main factors of aquarium maintenance are as follows. 

> Glass - keep it clean. Glass is a fantastic surface for green algae to grow on. The longer we leave the glass unkept, the more difficult it will be to remove the algae. The glass should be cleaned every few days using a clean sponge and an algae magnet (or an algae scrub). Simply wipe the internal sides of the glass, and keep on top of it. This needn’t be difficult!  

> Substrate - remove decaying organic matter. The substrate is a fantastic dirt trap. All uneaten fish food and fish waste usually end up trapped here, hidden in the gravel. Failure to remove this can lead to dangerous anaerobic bacteria and ammonia/nitrite spikes. The easiest way to remove decaying matter is by using a gravel cleaner. This should be done weekly, and the aquarium should also be topped up with fresh, dechlorinated water. 

> Filters - the media needs washing. As we mentioned earlier in this blog, a lack of flow through filter media can starve bacterial colonies of oxygen. A lack of water through the filter (and therefore reduced water surface disturbance) can also starve fish of oxygen, so it is vital filters are kept clean. We advise cleaning filters 1 to 4 times a month, depending on the size of the filter and fish stocks. Remove the media and wash it in a bucket of old aquarium water (removed from the gravel clean/water change). This will ensure the bacteria colonies are not harmed. And remember, we don’t need the filter media to be immaculate -we just need to remove most of the dirt and improve flow through the filter.  

Click here to read a more in-depth blog on aquarium maintenance.

Mimic their environment

All fish live in a particular habitat and will therefore have specific requirements. Do your research before purchasing a fish species to ensure your aquarium will be a suitable home. 

Shops can also advise you on this, so anything is achievable! To keep our fish happy we should ensure the aquarium reflects their biological needs.

The best freshwater water fish for aquariums 


Goldfish (Carassius auratus) are the classic aquarium fish. They will tolerate temperate and tropical conditions and are generally kept with other goldfish. They come in a variety of colours and body morphs (e.g.Orandas, Ranchus, Fantails, and Lionheads). Goldfish, like most ‘pond species’, prefer hard water conditions with a pH between 7-8. 

Community fish

Community fish are a species that can live peacefully with other species of fish. They also often have fairly generalist requirements. Community fish are also not aggressive by nature, which makes them an extremely popular choice in tropical freshwater aquariums. 


Livebearers is a general term for fish which produce live young (i.e. not in an egg). The most common livebearers are mollys (Poecilia sphenops), platys (Xiphophorus maculatus) guppies (Poecilia reticulata) and swordtails (Xiphophorus helleri). 

They are extremely popular in tropical aquariums as they’re easy to care for, easy to breed and come in a large array of body morphs and colour variations. Livebearers are some of the most colourful tropical fish available! All livebearers prefer temperatures of 24-27°C and a pH of 7-8. 


Tetras belong to the Characiform family of tropical fish, which has around 150 species to choose from! 

Tetras are hardy tropical fish which can adapt to and tolerate most aquarium conditions. They are small in comparison to other tropical species, and often like to shoal in groups of 10 or more.

The most popular species of tetra include Neon tetra (Paracheirodon innesi), Cardinal tetra (Paracheirodon axelrodi), Rummynose tetra (Hemigrammus rhodostomus), Glowlight tetra (Hemigrammus erythrozonus) and Black Phantom tetra (Hyphessobrycon megalopterus). 


Rasboras are schooling tropical fish which belong to the Cyprinidae family, and are therefore closely related to barbs, danios, goldfish and carp. Harlequin rasboras (Trigonostigma heteromorpha) are by far the most popular species of rasbora, as they are incredibly peaceful and easy to care for. Other popular species include Chilli rasboras (Boraras brigittae) and Scissortail rasboras (Rasbora trilineata).


Indigenous to Asia, gouramis are a well-liked species of medium to large-sized tropical fish.

There is a fantastic selection of species which are incredibly peaceful and display some striking colours. The most popular varieties include Blue gouramis (Trichogaster trichopterus), Dwarf gouramis (Trichgaster Colisa lalia), Kissing gourami (Helostoma temminckii) and Pearl gourami (Trichogaster leeri). 


Angelfish (Pterophyllum scalare) are a true staple of tropical aquariums. If you’ve ever been to an/aquatics shop, you will have almost certainly seen an Angelfish. These fish are beautifully unique, and renowned for their unusual, triangular body shape, sparkling scales, and rich colouration. So, it’s no wonder they are so popular! 

Native to South America, Angelfish grow to around 8” in size, so they’re larger than most other community fish. Angelfish are generally peaceful, however, they can become aggressive and predatory to smaller fish when they become large and mature. They like to live in fish tanks with several others of their own species, and shouldn’t be kept with any species which nips or bites fins, such as barbs. 


Danios are extremely hardy fish which can tolerate both temperate and tropical conditions. They are incredibly active and easy to care for, making them a great choice for beginners! 

The most popular species are Leopard and Zebra danios (Danio rerio), Giant danios (Devario aequipinnatus) and Galaxy rasboras (Danio margaritatus), which, despite the name are danios!  


Loaches are a fun addition to any tropical aquarium. They are peaceful scavengers who spend their time at the bottom of the aquarium, searching the substrate for leftover food. They act as a sort of basic clean-up crew for your aquarium! 

Most prefer to be kept in small groups and dislike a solitary existence, which can even kill them. The most popular loach species are the Clown loach (Chromobotia macracanthus) and Yoyo loach (Botia almorhae), which are great for eating pest snails. Other popular loach species include the Weather loach (Misgurnus angullicaudatus) and the Kuhli loach (Pangio kuhlii).


Freshwater sharks are becoming increasingly popular in tropical aquariums. And no, they’re not related to marine sharks! However, you can see where they get their name from.

All of them are highly active fish with large pointed dorsal fins and forked tails, renowned for occupying the middle and the bottom portion of the tank… sound familiar?

Freshwater sharks are easy to care for and will breed in captivity if given suitable space. The most popular varieties of sharks are Red-tailed black sharks (Epalzeorhynchos bicolor), Rainbow sharks (Epalzeorhynchos frenatum) and Bala sharks (Balantiocheilos melanopterus). 


Rainbowfish are a tropical freshwater species, native to Australia, Papua New Guinea, and parts of Indonesia. They love planted aquariums and prefer to live in shoaling groups. 

They are placid fish, making them a fantastic addition to community tanks. They are also very colourful and easy to care for, making them a fantastic choice for all levels of hobbyists. 

One of the most recognised rainbowfish types is the Bosemani rainbow (Melanotaenia boesemani), which has two tone colours of blue and orange. Other popular varieties include Threadfin rainbowfish (Iriatherina werneri), and Neon rainbowfish (Melanotaenia praecox). 


Killifish are some of the most colourful tropical fish available! They’re also generally quite small, which makes them ideal for nano aquariums. 

Killifish are found worldwide in tropical freshwater systems. They’re very closely related to livebearers, such as guppies and mollys, however, Killifish are egg layers by contrast. There are over 1,250 species of Killifish which have adapted to live in a wide range of habitats.

Some are easy to care for, while others are not, therefore it is important to do your research before purchasing a species. Our favourite species include Striped panchax (Aplocheilus lineatus), Blue glaris (Aphyosemion sjoestedti), Blue lyretail (Fundulopanchax gardneri) and, of course, the American flagfish (Jordanella floridae).


Betta splendens, also known as Siamese fighting fish, are tropical fish native to Southeast Asia. 

They’re extremely popular in the hobby and it’s not difficult to see why – they come in beautifully vivid colours and have incredibly elegant flowing tails. Bettas like neutral water (i.e. pH 7) and a protein-rich diet, such as frozen blood worms or brine shrimp.

It is a common misconception in the hobby that bettas will happily live in a nano aquarium, however, they much prefer the space to roam. So, a 45-litre aquarium would be the smallest we would ever recommend for a betta, but bigger is even better. 

They are not powerful swimmers and do not like too much flow in the aquarium, however, it must be suitably filtered as they’re also sensitive to ammonia and nitrite – sponge filters are ideal for this. Male Bettas cannot live with other male bettas, however, it is possible to introduce a harem of females. Equally, they will live comfortably with placid community fish that don’t bite or nip fins, such as neon tetras, Corydoras and Gouramis.


The most typical species of minnow kept in aquariums is the White Cloud Mountain minnow (Tanichthys albonubes). White clouds are native to China and Vietnam and grow to a length of around 1.5”. 

This species can tolerate both cooler temperate and tropical water temperatures. They are extremely peaceful fish and should be kept with other species of small fish. If you keep them with any fish which are considerably larger, such as goldfish, they are likely to become prey. 


Barbs are lively, hardy colourful fish which belong to the carp and minnow family, Cyprinidae. They are schooling fish, which are best kept in groups of over five. 

Barbs are generally extremely active and sometimes boisterous, so they should be kept with tank mates who can tolerate this. Most species of barbs will nip the fins of other fish species, so it is incredibly important you do not house them with any long-finned fish, such as Siamese fighting fish or guppies - as they will often kill them. 

Providing you follow these rules, barbs are still considered to be community fish. Our favourite species of barbs include Tiger barbs (Puntigrus tetrazona), Rosy barbs (Pethia conchonius), Golden barbs (Barbodes semifasciolatus) and Tinfoil barbs (Barbonymus schwanenfeldii).


Corydoras are a species of tropical catfish from South America, which are best known for living on the bottom of fish tanks. 

They are a beloved member of the fish tank clean-up crew and spend their time sifting through the substrate for morsels of uneaten food and debris, which reduces gravel cleaning to an extent. 

Corys are extremely placid, social fish which are happiest in shoals of 6 or more. Ideally, they should be kept in fish tanks with other small-medium community fish. Our favourite species of Corydoras include Bronze Corydoras (Corydoras aeneus), Albino Corydoras (Corydoras aeneus albino), Pepper Corydoras (Corydoras paleatus) and Julii Corydoras (Corydoras julii). 


Plecostomus are a species of South American catfish, best known in the hobby for sucking onto your aquarium glass and aquarium decorations, where they graze algae. 

They are prehistoric-looking fish which have armoured plates and spines on their body, designed to protect them against predatory fish. Also known as suckermouth catfish, plecos are a typical member of any fish tank where they act as part of the clean-up crew. 

While the common plecostomus (Pterygoplichthys pardalis) is the typical pleco found in aquariums, there are more than 150 species that include stunning variations with vibrant colours and unique patterns, referred to as L-numbers.

Our favourite, most unusual varieties include L-418 Royal plecostomus (Panaque nigrolineatus), L-128 Blue Phantom plecostomus (Hemiancistrus sp) and L-018/L-085/L-177 Gold-Nugget plecostomus (Baryancistrus xanthellus), but, be warned - most plecos grow large up to 24”). So, if you’re planning on purchasing this type of fish, make sure you have a suitably sized aquarium. If you’re looking for a smaller species for your aquarium, bristle nose plecos (Ancistrus cirrhosus) grow to around 4-5”.

Find all the fish you want for your aquarium at Complete Koi and Aquatics. 

Here at Complete Koi and Aquatics, we are proud to say that we are stockists of some of the most incredible tropical fish. 

Our extensive selection of freshwater fish caters to hobbyists of all levels, featuring both common and unusual species. So, if you’re looking for the best variety of healthy tropical fish, make sure to check out the rest of our website today. 

If you’re looking for advice on adding freshwater fish to your aquarium then this blog is a great guide, however, if you do have a question please don’t hesitate to reach out to us directly. Click here to get in touch today!

Shop the best aquarium products & accessories for your freshwater fish at Complete Koi today

For more information and advice on aquatics, check out the rest of our blog… 

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