Pond Water, pH, and Alkalinity (Hardness) in Ponds

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Water is a highly complex chemical and exhibits a range of properties no other fluid possesses. Water will dissolve almost anything albeit sometimes in very minute traces (witness the vast number of elements contained in sea water).
On its course to the sea water picks up minerals from the type of ground the river flows over.
In a barren granite type mountainous area (eg Scotland) water will tend to be weakly acidic (pH less than or close to 7) and will contain few nutrients useful to aquatic growth and therefore insect life. On the other hand rivers flowing through chalkly or limestone regions will pick up significant traces of calcium and other minerals and the water will become a lot more viable as far as aquatic life is concerned.


The best trout streams in the world occur in such regions (referred to as chalk streams or limestone rivers)... consider the world famous River Test in Hampshire in the UK. This kind of water is hard and has a high alkalinity. Water is also very clear by virtue of the very high mass of aquatic plants carried by the water. Remember algae is a plant that grows by photosynthesis.

Alkalinity is, like pH, a highly complex property of water and is a function of the elements dissolved in the water. Try to understand high alkalinity as being a buffer to pH changes... in other words pH will change over a much lower range and will tend to be more constant. Alkalinity is good for pond water.

The lack of alkalinity in ponds containing lots of suspended algae on sunny days can result in high pH and ammonia poisoning of the fish in a pond .


One of the very complex properties of water is pH and this is especially so in a pond where pH will change with time of day, source of water, amount of algae (and plants) in the pond and so on.

pH variations are normal and pH is also very difficult to measure accurately even in the laboratory let alone on the side of the pond using a low cost test kit.

My advice is always be very wary of pH testing and if you do insist upon testing for pH then do it twice a day early morning and late evening and plot the results so you can observe trends. A single pH result is unlikely to be meaningful and can result in completely the wrong action being taken.

Jane's Pond and Alkalinity... click the images to enlarge

This matter came up in an email I received from Jane recently.

I have provided my comments made to Jane and while it's impossible to be absolutely correct in the absence of more information my views are based upon some basic water property fundamentals as described above.

Jane sent me some pictures of her pond and the pond looks in excellent condition based upon the photographs.


Hi Tony,
I have been getting your articles for ages and have forwarded them to many of my ponding friends.

My husband and I expanded our hard liner pond recently and the new rubber liner pond has a lot more limestone in it than our old one so now we are having problems with too much alkalinity. We have tried Ph Down products and they work temporarily but the lime leaches back into the water. Is removing the majority of the stone our only solution?

Hope you have a suggestion.

Jane... Clinton, IA

Hi, Jane… it’s very difficult to have too much alkalinity. Why are you worried?… what problems are you seeing? Most ponds suffer from not enough alkalinity. Be careful when adding pH correctants

Hi Tony,

Well, the fish are very sluggish and not adapting well. The test kit put the Ph up over 9 and the algae has started growing like crazy. I found a couple of websites that suggest putting in some plaster of Paris blocks, which I have used in the past to keep the water balanced with good results but on a much smaller scale. I attached a few photos for you to see sort of the size and layout.
Thanks Tony for replying, I wasn't sure if it would reach you or not. I love your material!

Hi, Jane Your pond looks great.
It is MOST unlikely your true pH is 9 (fish would probably be dead from ammonia poisoning if this was true)… ammonia at pH more than 8.5 is deadly. Measure the pH early morning (preferably at a time close to dawn) and again at end of day and let me know results. If you can use 2 different test kits.
The water looks great. High alkalinity does produce rapid growth of aquatic plants and algae such as the type attaching itself to surfaces will grow well under these circumstances… limestone streams are very fertile and generally hold the biggest trout for example because the conditions favour aquatic growth and thus prolific insect life. On the other hand streams with low alkalinity produce small stunted fish on the whole because there is not enough food to sustain a growing population.