Understanding Water Hardness in Ponds & Aquariums
Many new pond and aquarium hobbyists aren’t aware of GH & KH (or water hardness to put it simply), and as a result don’t understand the vital role it plays in fish health.
Water Hardness Explained: What It Is and Why It Matters
GH or ‘General Hardness’ refers to the dissolved levels of magnesium and calcium ions in the water, while KH or ‘Carbonate Hardness’ refers to the measure of carbonates and bicarbonates dissolved in the water.
- KH (carbonate hardness) helps stabilise pH levels by buffering acids, preventing sudden swings that can stress or harm fish.
- GH (general hardness) measures the amount of calcium and magnesium, essential for fish health, bone development, and osmoregulation.
Together, GH and KH support a stable, healthy environment in both ponds and aquariums
Naturally occurring water in the form of streams, rivers, ponds, lakes etc. is rarely purified in the distilled water sense. Instead it is packed full of dissolved salts, buffers and nutrients, and the levels of these will vary according to geography and local conditions.
Fish of all types worldwide, have evolved over millions of years to inhabit very specific water conditions, and it is our job as pond/aquarium keepers to try and replicate these conditions in order to keep the fish happy and healthy.
For those with limited knowledge or eagerness to keep exotic fish, the easiest approach is to select fish which are suitable for the local (dechlorinated) tap water. Alternatively, advanced aquarists or pond keepers may choose to match the water chemistry to match the selected fish’s requirements.
Not everyone however is blessed with perfect tap water…
In certain areas of the UK where the water is particularly soft, lacking any real mineral content (such as Manchester), pond and aquarium hobbyists may be forced to buffer their water, introducing minerals in the form of GH & KH in order to make it suitable for fish keeping.
Balancing GH, KH, and pH for Healthy Aquatic Life
GH and KH, are often intertwined with pH (a scale used to specify how acidic or alkaline a water-based solution is), which is another measurable factor in water chemistry, equally important to fish health.
Although pH, GH & KH are separately measurable entities, it is difficult to adjust one parameter without affecting the others, which will be further explained below.
pH Basics: Why It Matters and How to Control It
A pH of 7 is considered to be neutral while pH’s below 7 are considered acidic and pH’s above 7 are alkaline. To a fish keeper in any aspect (pond or aquarium), two aspects of pH are important:
- Rapid changes in pH can stress fish out and be detrimental to fish health. As a result, the pH of your pond or aquarium should remain relatively consistent and stable.
- Fish are adapted to specific water conditions, so it’s important to match your pond’s pH to their needs. Most species thrive in neutral water with a pH between 6.5 and 7.5. Always check your fish’s preferred parameters before adding them to ensure a suitable environment.
How KH Influences pH Stability in Aquatic Environments
KH is vital for stabilising pH by buffering acids produced during the nitrogen cycle. Without sufficient KH, water can become acidic over time, which may harm fish.
In hard water areas, KH occurs naturally, but in soft water regions, such as around Manchester, it often needs to be added to maintain pH balance. We use our own brand KH Buffer in store for all our ponds and aquariums and find it to be highly effective in increasing the KH and Ph readinng rapidly.
Fish like Malawi Cichlids require higher KH levels and alkaline water (around pH 8.3), whereas species like Discus prefer low KH and slightly acidic water (around pH 6), making KH levels critical depending on your fish.
Balancing GH and KH for Optimal Fish Health
Maintaining appropriate levels of GH and KH creates permanent hardness, which aids most fish in osmoregulation. Osmoregulation is the process by which fish regulate the balance of water and salts in their bodies to maintain stable internal conditions—similar to how humans drink water to stay hydrated.
KH (carbonate hardness) is responsible for buffering the water, helping to keep pH levels stable and preventing sudden fluctuations that can stress or harm fish. A steady pH is crucial for both fish and beneficial bacteria in your filter system.
GH (general hardness) measures the levels of calcium and magnesium in the water—minerals that are essential for fish health, aiding in bone development, enzyme function, and osmoregulation (how fish control their internal water balance).
Maintaining proper KH and GH levels in your pond or aquarium creates a more stable and healthy environment for fish, plants, and filter bacteria alike.
For example, Koi Carp prefer harder water with higher GH levels and may require periodic GH supplementation if the tap water is naturally low. GH minerals are consumed by the fish and also lost during regular water changes.
Most tropical fish tanks prefer moderately hard water, and just like Koi keeping, may need this introducing in small doses of aquabuffer or re-mineral buffer, periodically, if your tap water is naturally soft.
Several species of South American fish such as Discus actually live naturally in soft water and so prefer low GH conditions. If you live in an area of the country with hard water, but want to keep soft water fish, there are several products on the market which can help reduce levels of GH in the water.
Not sure about your water hardness? Test it now for peace of mind.
If you’re unsure about your local water hardness, we would always recommend testing. This can be done at home or by our team of in-house professionals at Complete Koi & Aquatics. We can then follow this up with any advice required for fish keeping.
For any questions related to the above, please don't hesitate to contact Complete Koi & Aquatics:
Tel: 01204 582105
Email: info@completekoi.com
Address: Complete Koi & Aquatics, Unit 4, Tonge Bridge Way, Bolton, BL2 6BD
