Alkalinity - KH - and its relation to pH.


Alkalinity is viewed slightly differently in the general filtration and fish worlds.
Total alkalinity in U.K. drinking water reports includes more than we usually test for in the fish world. But there are more similarities than differences so total alkalinity in a water report is the nearest thing to alkalinity - KH - that we know as fishkeepers.

KH stands for carbonate hardness. It measures the carbonates and bicarbonates in the water. This is often called temporary hardness as it can be removed by heating. We can see domestic examples of this e.g. the scaling of kettles and furring up of domestic boilers in hard water areas.

KH is extremely important to fishkeepers as it acts as a pH 'buffer' or 'stabiliser'.

pH measures acidity and alkalinity - a low pH denotes an acid water, a high pH denotes an alkaline water and a pH of 7 is neutral.

This pH range has led fishkeepers to think that water with a low pH is soft and water with a high pH is hard. That assumption makes a sort of fishkeeping sense when we think of breeding discus kept in soft acidic water (low pH and KH) and marines kept in hard alkaline water (high pH and KH).

Unfortunately that isn't the reality of water and, in some ways, it is a misleading assumption. It is not misleading when looking at the lower end of the scale as soft acidic water will have low pH and KH values.

But it can be a misleading view if an assumption is made that all waters with a high pH are automatically 'hard'. A classic example of this is North-East Pennine, upper moorland water. This water is very soft but it is also very alkaline. So in this instance you find a naturally very soft water, which is also very alkaline and has a high pH value of up to 9+.

KH - carbonate hardness is extremely important. It acts as a pH 'buffer' or 'crutch' i.e. it stabilises the pH in the aquarium or the pond.

To understand the relationship of KH and pH in fish tanks we have to think in what seems an illogical way, in fishkeeping terms.

When low carbonate hardness (KH) in the fish water fails to buffer the pH adequately, the pH appears to react in unpredictable ways. The pH can drop to very low values or it can rise to very high values, or the pH can be so unstable that it zigzags between the two extremes (high and low).

To stabilise the pH of soft water the carbonate hardness of the water has to be raised.

So you raise the value of carbonate hardness in soft water to bring the pH and KH up to values your fish need. This may be taking a KH value up from 1 to 3 or 4 German degrees of hardness. It may be taking the pH from 6 to 7.0 or from 1 to 6 if dealing with reverse osmosis water. Raising the KH value of soft water, which has a low pH, will drive a low pH up.

The apparent illogicallity starts when handling very soft water with a high pH.

A few paragraphs ago I outlined that a soft water can have a very unstable pH. And the pH can go up as well as down.

This is where the paradox starts: When a soft water has a high pH, the pH can be brought down through adding carbonate hardness, so the pH is stabilised. That's right! On soft water with a high pH, you harden the water to bring the pH down to somewhere between 7 and 7.5.

The only way you can tell whether the carbonate hardness is at the right value in a fish tank is to measure it! Not all koi centres stock KH test kits but they are often obtainable from an aquarium shop and should certainly be available at a marine shop.

KH test kits sometimes contain guides as to the KH value you need for different aquarium fish. A good dealer will be able to advise you. The hardest type of fish to find the right information for is koi. You should aim to have a minimum KH value in a pond of 5.5 degrees German hardness (about 100 mg/l). If buffering up very soft water you may have to settle for a KH value of 4 (about 72 mg/l). Get it nearer that magical 5.5 mark in a pond if you can.

Don't assume that if you live in a hard water area that your koi pond will automatically have enough carbonate hardness in the water. Ponds with very large filtration systems, or tanks where only very small amounts of water are changed, or where rainwater has been stored to top up the pond can lead to the 'fish' water developing a low KH. I have seen this happen three times so far with ponds. I have also heard of many aquaria suffering pH crashes, where reverse osmosis water has been insufficiently buffered. KH testing should be a normal part of a fishkeepers routine water testing.

For aquarium use there are many commercial buffers on the market, which can be used to raise KH and stabilise pH.

For koi ponds the answer is simpler. Add a natural buffering agent such as coarsely crushed oyster shells or coarse calcified algae to the biological filters. Another option for very soft water is white dolomite marble chips. Testing the pond water for KH values will tell you if you have installed enough buffering agent.

P.S. I never recommend using sodium bicarbonate. It gives a lovely quick fix to the pH but did you know that it has a medium to long-term effect of driving the buffering agents out of the water? pH crashes in aquaria have been reported to be linked to its use.


Ann Telford, copyright, June 2001.
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