A Simple Guide to Marine Fish and Aquariums

When it comes to marine life, there are so many different aspects to consider. Which fish work well together? How do you set up a marine aquarium? How do you look after marine fish? In this in-depth guide, we will explore all of these areas below, including product recommendations that can assist with the upkeep of your tank.

What are marine fish?

Marine fish, to put it simply, are saltwater fish, i.e. fish which would normally live in the sea. Marine fish come in the most incredible colours, shapes and sizes and are extremely popular for aquarium hobbyists. 

What is a marine aquarium?

A marine aquarium is a saltwater aquarium which houses suitable inhabitants – i.e. anything which would normally live in the sea such as fish, crabs, shrimp, corals and other invertebrates. 

Are marine fish hard to look after?

In many ways the principles of a marine aquarium are very similar to any other tropical or coldwater aquarium. The key to a healthy marine aquarium is in keeping good water chemistry. In order to do this, testing and adjusting is required, covered further on in this blog. 

Providing you follow the basic principles of fish keeping and your water chemistry is good, most marine fish are relatively easy to care for. As with keeping any fish, some species will be naturally more challenging than others. They may have very specific space/dietary requirements and/or a lower tolerance for changes in water chemistry. Therefore, it is critical to research and prepare accordingly prior to purchasing. 

Why can’t marine fish survive in freshwater?

All life on earth is supported by water. In most instances the water interacts in some way with salt – too much or too little can be fatal. Living organisms balance their water levels through a process called osmoregulation i.e. the regulation of osmosis. 

Marine fish have evolved over millions of years to live in saltwater. In the case of marine fish, their body salt level is lower than that of the sea water (i.e. the seawater is hypertonic to marine fish). As a result, marine fish lose comparatively large volumes of body water to the surrounding seawater through osmosis. 

In order to maintain the required water levels, marine fish must therefore consume large amounts of seawater in order to avoid dehydration. Naturally, sodium and chloride ions (i.e. salt) end up in the fish’s blood. Marine fish have specialised chloride cells in the gills which remove both sodium and chloride ions from the blood and excrete them back into the water. This process constantly uses energy and is a vital equilibrium in keeping marine fish alive.

If a marine fish was therefore placed in freshwater, the body of the fish would take on far too much water causing the fish in question to swell and die very quickly. Marine fish can be very sensitive to the slightest fluctuations in the salinity, so it is important to understand the biological process before keeping a marine aquarium. 

What marine fish can I keep together?

If you’re just starting out with a marine aquarium, the following groups of fish are great for beginners:

  • Damselfish – This group of fish are on the smaller side making them a suitable fit for most aquariums, nano or large. They have a slightly aggressive and solitary nature, so it is important to include hiding spots/space. They come in an array of striking colours from bright yellow to deep blue. 
  • Clownfish – Arguably the most recognisable of all marine fish, Clownfish are very hardy and simple to care for. There are a number of varieties and colours but the most common is the False Percula which is orange and white.
  • Green Chromis – One of the few marine fish which will shoal when kept in captivity. As the name suggests, they are bright green in colour and look fantastic when kept in numbers. They have a very relaxed and mellow nature making them a fantastic addition to a marine aquarium. 
  • Royal Grammas – Bright purple and yellow fish which are on the smaller side, only growing to around 3 inches. They’re extremely peaceful, however they can be aggressive with their own kind so are best kept as singles. They actively seek out low light areas of an aquarium normally in the form of a cave where they will hide. 
  • Firefish – Long thin fish with a white, pale body and a striking red tail (hence their name). Firefish have an extremely gentle temperament making them a great fit for most marine community aquariums.  
  • Wrasses – These brightly coloured marine fish come in a variety of colours and like hiding in rocks towards the bottom of an aquarium. They generally have a peaceful nature but can be quite messy, so a larger aquarium is preferred. The best types of Wrasse for community aquariums are six-line Wrasse, flasher Wrasse and fairy Wrasse. 
  • Cardinals – The pyjama cardinal has a yellow/green face followed by a black and white body overlaid with red polka dots. Given their small size they’re suitable for both small and large aquariums. In larger aquariums they will school in numbers. They have a very peaceful nature and are most active during the night.
  • Blenny – These marine fish are very peaceful and have incredible personalities. They will spend the vast majority of their time hiding amongst the rocks, peering out at their surroundings. They can be territorial with each other so are best kept as singles. There are many different varieties of striped and horned Blennies, but our favourite is the Midas Blenny.    
  • Butterflyfish – There are many species of Butterflyfish and they come in a vast array of striking patterns and colours. As a rule of thumb most butterflyfish are peaceful and make a beautiful addition to marine aquariums. 
  • Angelfish – There are many species of reef safe Angelfish which will not eat corals and/or decorative invertebrates. Equally there are many which can cause issues in a marine aquarium particularly when corals are present. As with selecting any fish, research is paramount when selecting fish for compatibility. Our favourite Angelfish for community aquariums are Coral Beauties, Bicolour Angels, Fireball Angels and Koran Angels.
  • Gobies – There are many species of Goby and they can be a fantastic and placid addition to a marine aquarium. Our personal favourite is the sand sifting Goby, which actually provides a service to the aquarium by cleaning the sand with its mouth, expelling clean sand from its gills. 

Marine fish by nature are very territorial, so while these are a great place to start it is important the aquarium has suitable space, hiding spots and appropriate tank mates. Once again it is important to research before introducing new fish to an aquarium to check compatibility with other fish, corals and invertebrates. 

What marine fish can I keep in a nano tank?

When space is at a premium, the size of a fish species becomes incredibly important. You ideally want to select species of marine fish which stay small for nano aquariums. The other factor to consider in nano marine aquariums is water chemistry. When dealing with 50 litres or less, water parameters can change very quickly. An example of this is the salinity which will change on a daily basis as water naturally evaporates from the aquarium. Therefore, we also require relatively tough species which can handle changes in parameters such as salinity.

With this in mind, an ideal fit for a nano marine aquarium will be both small and hardy. We believe the below are a good fit for nano marine aquarium:

  • Clownfish (e.g. False Percula, Tomato Clown, African Clown)
  • Damselfish (e.g. Domino Damsel, Humbug Damsel, Blue Damsel, Regal Damsel, Lemon Damsel)
  • Royal Gramma
  • Pyjama Cardinal
  • Chromis (e.g. Green Chromis, Blue Chromis)
  • Firefish
  • Gobies

As mentioned previously, many marine fish can be territorial and aggressive in certain circumstances. Competition for space in small aquariums can often heightened this. Therefore, due to space constraints it is often best to keep smaller numbers of marine fish in a nano aquarium (e.g. 2-4).

Which marine fish are best suited for beginners?

The best fish for beginners are without doubt the hardiest varieties, which have a tolerance for changes in salinity, pH, ammonia and nitrite. We believe the best fish for beginners are therefore Clownfish and Damselfish. There are many species of both which come in just about every colour, and they’re incredibly simple to keep providing you follow the basics.  

How to set up a marine aquarium

There are many different aspects to consider when setting up a marine aquarium:


Lighting a marine aquarium can be very important. If you are keeping a fish-only aquarium, standard aquarium lighting will be sufficient. The moment that corals and other invertebrates, such as anemones, become a consideration, then a higher standard of lighting will be required. 

Our favourite marine lighting is the Hydra by Aqua Illumination. These are premium lights with infinite settings which can be fine-tuned to suit your aquarium. If your budget will not permit the Hydras, another great option is the marine spec light by Fluval. Once again, these lights come packed full of features and adjustable settings to suit your marine aquarium. 

In any instance, once the lighting is rigged up and suspended over the aquarium (in the hood, on a clip or on a suspended rail) do some research to find the optimum settings for the depth of aquarium you have and the species you are keeping. The settings for corals should closely reflect a true day, mimicking the sun rising and setting. The lights should come on in the morning and slowly ramp up to full power before steadily dimming down and turning off in the evening. Once a good setting has been identified, it is best not to alter this where possible. Corals and invertebrates can be incredibly sensitive to change. 


For marine aquariums, crushed coral sand/gravel is paramount in our opinion. This is the same type of sand found on coral reefs. It is generally rich in calcium and has a direct impact on water chemistry. Over time, trace elements such as calcium will leach from the sand into the water having a positive impact on the aquarium in question. Coral sand also increases the pH to a more suitable level for marine fish and can harbour good bacteria, which becomes useful in biological filtration. Sand can also provide a suitable home for certain species of marine fish which prefer to be buried in the sand. 

All sand should be washed prior to putting in the aquarium. This is generally done in a large bucket, which should be repeatedly filled with tap water, disturbed to turn the water cloudy and emptied until the water runs clear. Once the sand is nice and clean it should be put at the bottom of the aquarium to a depth of around ½” – 1”. 


When it comes to rock there are a few options:

  • The cheapest form of rock that can/should be put into a marine aquarium is dry Ocean Rock. This is white/cream rock, which is pitted with holes, ideal for fish to hide in. It is very dense and is ideal for creating rocky aquascapes or as a skeleton structure to support live rock. 
  • The next option in marine aquarium rock is dry live rock. An example of this is Life Rock by Caribsea. This is manufactured, highly porous calcium carbonate rock which comes preloaded with spored bacteria. Once wet, these bacteria will become activated, thereby turning the rock into a form of biological filtration. 
  • The final option is wet live rock. Wet live rock is true live rock which has been cultured in the sea to support vast amounts of bacteria, which is great for biological filtration. The bacteria present in the cultured rock helps break down ammonia and nitrite which in theory creates great water chemistry. Live rock can also host hitchhiker invertebrates such as crustaceans, anemones and worms which can make interesting additions to your aquarium. 

While hitchhikers can be interesting, they can also be detrimental to an aquarium. Invertebrates (such as nudibranchs – sea slugs) can cause issues in reef aquariums and likewise live rock can harbour bad bacteria such as cyanobacteria. To prevent this, it is best to get live rock from a reliable source.


When it comes to water in marine aquariums, it is best to use RO (reverse osmosis) water. Reverse osmosis is a water purification process which uses partially permeable membranes to remove ions, molecules and larger particles from tap water. The end result is pure water. Shops such as us here at Complete Koi & Aquatics sell RO water by the litre as both fresh or salted varieties. 

When starting a marine aquarium, you will obviously need to fill the aquarium with salted RO water. Fresh is only required when topping up a marine aquarium as a result of evaporation. RO water is the safe option for sensitive marine life such as corals and other invertebrates. 

How do I maintain my marine aquarium?

Routine maintenance is essential in any marine aquarium to keep it looking tip top. The most important maintenance is without doubt testing and maintaining good water chemistry. Regular tests should be conducted in the following areas:

  • pH should be tested regularly and should be kept between 7.6 - 8.4
  • Ammonia should be tested and kept at a minimum 
  • Nitrite should also be tested and kept at a minimum 

At the most basic level, the only other additional factor which must be carefully monitored in marine systems is salinity, which is the concentration of sea salt present within the aquarium water. This would normally be running at a SG (specific gravity) of 1.025 OR 35PPT. This can be measured using a hydrometer or a refractometer.

For more advanced reef aquariums, it is also important to test for water hardness/alkalinity, phosphate, calcium and magnesium. Salifert provides test kits for all of these. 

The next important routine maintenance to undertake in a marine aquarium is a gravel/sand clean. This is generally done when the substrate looks messy, on a weekly/bi-weekly basis. In order to do the gravel/sand clean you will need a suitable tool to create a siphon, such as the Betta Aquarium Gravel Cleaner

Put the gravel cleaner in the aquarium water and start a siphon with the waste pipe in a bucket to collect the water. The head of the gravel cleaner should then be poked into the sand, focusing on soiled areas to remove any large waste/debris which will end up in the bucket. 

When undertaking a gravel clean, you should look to remove 10-20% of the water from the aquarium. This should then be topped up with new salted RO water. The wastewater removed from the aquarium is an ideal solution to clean filter media in, as it is devoid of any nasty chemicals such as chlorine. 

To undertake a filter clean, remove soiled media from an internal/external filter and wash this in the bucket of aquarium water. You are not looking for filter media to be spotless, but instead quite clean. Filter media harbours good bacteria and we do not want to get rid of this.  

The final task to undertake in routine maintenance of a marine aquarium is glass cleaning and spot cleaning. For glass cleaning, a course sponge is best used to remove algae. This may be a new/clean kitchen sponge however useful tools are available to assist in this. For hard to reach areas, Fluval glass cleaning kits are a great accessory to have.

Another useful tool in removing algae are magnet cleaning accessories. Our personal favourite is the Fluval Razor, which comes loaded with a cleaning sponge and angled razor blade for easy removal of algae.

Spot cleaning is the cleaning of rocks and other ornaments with heavy algae build up. Ornaments which can be easily removed should be removed and washed clean in tap water. For rocks and other ornaments which can’t be removed, it is best to use a toothbrush to remove heavy algae build up. Some algae in the aquarium is absolutely normal but excessive amounts can cause issues and make the aquarium look less desirable.

What should I feed my marine fish, and how often?

Different marine fish have different diets. Marine food comes as dry pellets/flake, dried seaweed, frozen food or fresh/live food. The species you plan on keeping will determine the type of food required and research should be done prior to purchasing fish. 

At Complete Koi & Aquatics, we like frozen food as it is very nutritious and is relatively hassle free. Frozen food comes in a variety of forms, but the most common type is brine shrimp, which most marine fish will take. We recommend defrosting the frozen food in a small container of warm aquarium water. Once defrosted, this should be then distributed around the aquarium giving all the relevant organisms a meal. 

You will know when the marine fish have had their fill as they will generally stop eating. Marine fish generally eat more than their tropical/cold water counterparts. They expend lots of energy in osmoregulation and thus should be fed once or twice a day. 

Can marine fish eat bloodworm?

Marine fish can and will eat bloodworm, however it is not recommended for them. Bloodworm does not feature in the natural diet of a marine fish and will therefore not provide adequate nutrition. Instead, feed them shrimp such as Mysis/brine shrimp.

Which marine fish eat algae?

The most common cause of algae in marine aquariums is down to poor water sources. Tap water is often saturated with silicates, phosphates, nitrates which algae thrives off. To prevent algae in the first place, it is important to use good RO water and keep excess nutrients to a minimum. Marine fish can be used for algae control, but invertebrates are also helpful in the fight against algae. We believe the below species are the best for combating algae buildup:

  • Sand-sifting Goby – The sand-sifting Goby is a fantastic cleaner. It will readily consume large amounts of sand, expelling clean sand through its gills while consuming any food sources (such as Algae) hidden within. Algae grows naturally on sand, so to keep the sand in pristine condition a Goby is highly recommended. 
  • Kole Tang – Kole tangs are small, peaceful tangs which are reef safe. They naturally consume film algae, hair algae and macro algae making them a great part of the clean-up crew. 
  • Fox face (Rabbitfish) – The fox face is another formidable algae eater. They will readily chew up hair algae and eat almost all macro algae. They’re a very hardy and colourful herbivore making them a super addition in marine aquariums. 
  • Nassarius Snail – Nassarius snails don’t actually consume algae, but they eat left over detritus which keeps excess nutrients in control, limiting algae growth. 
  • Turbo Snail – The turbo snail is the ultimate algae eating invert for a marine reef aquarium. They’re extremely peaceful and will happily mow down several varieties of nuisance algae. We recommend several turbo snails in almost any marine aquarium. 

As with any helpful cleaning fish/invert species, they will do a good job, but some level of manual input will also be required. Be prepared to use a sponge, algae magnet/razor or toothbrush to clean the aquarium on a frequent basis. 

What is the life expectancy of marine fish?

The life expectancy of a marine fish varies from species to species. On average we would say most medium sized marine fish live 10 – 15 years, but there are obviously exceptions to that rule. 

Smaller fish such as gobies, blennies and pipefish/seahorses often have a shorter life span of around 5 – 6 years, while clownfish and tangs can often live well into their 20’s. At the opposite end of the spectrum, certain species of grouper can reach 50 years of age. 

What is the ideal temperature for a marine aquarium?

The ideal temperature for a marine aquarium is 24 – 26 °C.

How do marine fish regulate osmotic pressure?

Marine fish have an internal osmotic concentration lower than that of the surrounding seawater, so naturally they will lose water and gain salt through osmosis. Marine fish have to counteract this by ingesting lots of water, while excreting excess salt, through specialised cells in their gills, thereby regulating their osmotic pressure. 

What is velvet disease and how does it affect marine fish?

Velvet disease is one of the most common diseases that affects marine aquarium fish. It is known by several names including amyloidosis, oodinium, and gold dust disease. The organism which causes velvet disease is a single celled dinoflagellate known as Amyloodinium ocellatum

Velvet disease can be fatal to a marine aquarium if left untreated. Velvet disease attacks the skin and lungs of a fish – early signs include gold coloured spots on the fish’s skin and the fish infected will often be breathing rapidly. As the disease takes hold of a marine fish, it will lose the ability of transferring oxygen across the gill membrane, leading to suffocation. 

Velvet disease is most commonly treated with copper. Please note, copper treatments should be used at very specific dosages, so follow bottle instructions carefully to avoid unnecessary fatalities. Copper treatments are also highly toxic to invertebrates and should not be used in reef aquariums. 

What should the phosphate levels be in a marine aquarium?

Phosphate (PO4) is a naturally occurring trace element found in sea water. The ideal level of phosphate in a marine aquarium is an immeasurable one, i.e. 0 – 0.05ppm. In order to test for phosphates it is important to use a reliable test kit – our favourite is the Salifert Phosphate Test kit.

Phosphate is a primary nutrient source for algae growth. When phosphate levels increase, algae blooms become a common occurrence. Aside from looking messy, it can also have adverse effects on corals. Phosphate is most frequently introduced into aquariums through poor water. It is recommended to add high quality RO water into marine aquariums to prevent increased levels of Phosphate, which occurs naturally in tap water.

Phosphate levels can be managed in marine aquariums, in a number of ways:

  • Control it at the source. As mentioned above, only add high quality RO water to a marine aquarium, which should contain negligible amounts of Phosphate. 
  • Food – There are varying levels of phosphate in fish food. Use high quality marine fish food and feed sparingly. Overfeeding/uneaten food is a source of phosphate.
  • Pellet Reactors – Highly efficient in the removal of unwanted Phosphates. Pellet reactors sit in a marine sump and work by tumbling bio-pellets, which act as a food source for nitrate/phosphate converting bacteria. Our favourite pellet reactor is the Reef Bio-React by Tropical Marine Centre.
  • Algae Refugium – An area of the sump can be lit and converted to grow types of tropical seaweed such as Chaetomorpha (Chateo). Chaeto will naturally soak up Phosphate from the aquarium and should be routinely harvested thereby exporting any unwanted nutrients. 

Marine aquarium water quality parameters

When keeping marine aquariums, we recommend following the below water quality parameters:


  • Salinity - SG (specific gravity) of 1.025 OR 35PPT. This can be measured using a hydrometer or a refractometer
  • pH - Should be kept between 7.6 - 8.4 
  • Phosphate – Should be kept between 0 – 0.05ppm

Additional Testing for Reef Aquariums

  • Calcium – between 400 - 450 ppm 

What is reverse osmosis water?

Reverse osmosis is a water purification process which uses partially permeable membranes to remove ions, molecules and larger particles from tap water. The end result is pure water. Complete Koi & Aquatics (and many other aquatics shops) sell RO water by the litre and it comes in both salted and freshwater variations.  

How can reverse osmosis water help my marine fish? 

As mentioned, reverse osmosis produces high purity water free of chlorine, water hardness, heavy metals and other elements/toxins. This allows you to design precise water conditions for your marine aquariums regardless of the local tap water conditions. Certain trace compounds can have adverse effects on both fish and invertebrates, so using RO water gives you the best possible starting point for marine aquarium water.

Why does my marine fish have a cloudy eye?

Cloudy eyes in marine fish may be a sign of a bacterial infection. Harmful bacteria can manifest in aquariums as a result of poor water quality of injured fish. Bad bacteria can then enter fish and in certain circumstances, migrate to the eyes giving them a cloudy appearance. In order to prevent cloudy eye, it is important to do the following: 

  • Remove detritus or anything dead from the system – This will act as a bad bacterial trap.
  • Add active Carbon to the filtration.
  • Clean filters thoroughly in aquarium water.
  • Conduct weekly water changes of 10-20% during a gravel clean. 

If possible, it is a good idea to have a hospital tank for injured / infected fish, where appropriate treatment/care can be provided away from the main aquarium. 

Why is my marine fish jumping out of the tank?

All marine fish have the ability to jump out of an aquarium under certain circumstances. The most common reason for fish jumping from an aquarium is due to unsuitable tank mates. Fish will naturally jump out of the water to escape predation. In the wild, they will naturally fall back into the water, but at home this can often end in fatality. Ensure that there are no fish in the aquarium which could predate other fish, and this will reduce the chances of fish jumping. Likewise, suitable hiding places can be effective in reducing fish jumping. 

Other reasons why fish may choose to leap from water are as follows: 

  • Poor water quality - When there is a build-up of nitrate and ammonia, this irritates the fish. A natural response to this is leaping from the water. 
  • Limit sound and movement outside the aquarium – Fish over time will become accustomed to human activity outside of the aquarium. New fish can be particularly skittish, and loud noises and/or excessive activity (such as children banging on the glass) can cause fish to jump. 
  • Dusk to Dawn – In the wild, the light increases and fades gradually as the sun rises and sets. In many aquariums this is not possible, and the sudden switching of light can cause fish to jump in defence. Where possible it is best to use gradual lighting to reduce the chances of this. 

In any circumstance the best way of reducing fish jumping onto your floor is by using a glass /acrylic or mesh cover. It is however important to understand the cause of the jumping in any situation, rather than addressing the fish reaction. A jumping fish is normally a distressed fish. 

Where is the best place to buy marine fish? 

There are many suppliers of marine fish across the UK, but at Complete Koi & Aquatics we have a true passion for gaining a deep understanding of marine fish. Visit us in store today to view our gorgeous varieties, or even just pop in for some advice!

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