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pH and the Fresh Water Aquarium


The pH scale goes from 0 to 14. When the pH is 7 the water is neutral which means it is neither acidic nor alkaline. If the pH drops below 7 then the water is acidic and the lower the pH the more acidic the water. If the pH is above 7 then the water is alkaline and the higher the number is above 7 the more alkaline the water is.

The pH scale is logarithmic and each unit of represents a change by factor of ten. So pH 5 is ten times more acidic than pH 6 and pH 4 is one hundred times more acidic than pH 6 and so on and so what may appear quite small changes in pH are in fact quite large changes and such large changes will stress any fish present.

aquarium ph

Adjusting the pH

Altering the pH of aquarium water must be done gradually and in a controlled manner, filtering through peat in the filter isn’t controlled and in water with a low carbonate hardness it could cause the pH to crash. If you want to use peat or any other ‘natural’ product to adjust the water a better method would be to place a small filter containing the peat in a bucket of clean water, filter the water so that the pH has dropped as far as it will go and then use this water as an additive where the amount and rate at which it is added can be fully controlled. The same care should be applied when adjusting the pH upwards.

The ideal time to adjust the pH is when carrying out partial water changes. This means that the new water entering the aquarium is already at the ideal pH and it keeps the conditions within the tank very stable. There are many commercial products for both raising and lowering the pH and these offer the simplest way of making any necessary adjustments.

If slightly acidic water is required but your tap water has a very high carbonate hardness it may be necessary to dilute the tap water with either rain water or r/o water first in order to lower the carbonate level (KH) or it will take an awful lot of a pH lowering product to make any difference to the pH.

pH and fish keeping

Some fish have been in the hobby for many decades. So long that they are now virtually domesticated and probably wouldn’t survive for very long in the wild. These domesticated fish have over the years been acclimatised to live in water which is fairly neutral and quite different to the water of their ancestors. But other fish which are still fairly new to the hobby, wild caught fish or fish which come from the more extreme environments like Rift Valley Cichlids prefer the same water as their wild counterparts and so it is important that a little research is carried out about the fish that you intend to keep. Wild caught fish normally sell for a premium and will almost certainly be marked as wild caught fish in the shop.

Almost all fish live in the range from pH 5.5 to 9 with the majority living in a more narrow range of 6 to 8.5. [sws_pullquote_right] Each species of fish has a physiology which restricts the pH range it can thrive in and if these limits are passed it could cause the rapid demise of the fish. [/sws_pullquote_right] Most fish will have an ideal range for pH and most will have a little leeway but there are limits.

The pH of aquarium water is not static, carbonates are used up during the biological processes which are constantly going on in an aquarium, calcareous rock or gravel will slowly dissolve and have an impact on the pH these things do occur in nature but changes take place very slowly in most cases unlike the changes which can take place in the aquarium which can occur in the blink of an eye. Avoid using calcareous substrate and rock work in soft water aquaria.

In order to prevent problems associated with a pH crash it is recommended that the carbonate hardness be maintained around 100mg/l KH 5 to 6.

3g/l of bicarbonate of soda will raise the KH value by one.

Which pH is best for my (captive bred) fish

Most commercially bred fish which have been a long time in the hobby will have been bred over many generations in water which is fairly neutral and regardless of where their ancestors came from fairly neutral water suits them best. A pH range of 6.5 to 7.5 is safe for most fish but their are exceptions. Some species of Molly still demand hard alkaline water or their health suffers. It is important to make a proper check on the requirements of any species that you intend to keep BEFORE getting the fish.

pH and fish health

General symptoms of being in the wrong pH include lethargy, an increase in mucus production and respiratory distress. There are two main conditions which are caused as a direct result of keeping fish in water of the wrong pH.

Acidosis: this occurs when the pH is to low and beyond the range where the fish can cope. If a fish is exposed to very acidic water beyond where its body can cope the gills degenerate and become coated with thick mucus as the fish tries to protect them. This causes asphyxiation and rapid death of the fish.

Alkalosis: occurs when the pH is to high and beyond the fish’s ability to cope. A fish which normally lives in fairly acidic conditions will have evolved mechanisms to cope with those conditions, their gills will be adapted to extracting enough salt from the softest water in order to maintain it’s body chemistry. When a fish like this is exposed to very alkaline conditions it will be unable to excrete ammonia from its gills which will result in its eventual death.

About Andy Rapson

I've been interested in fish for about fifty years and I have kept many different species in that time. I have also worked in the fish trade running my own fish shop and I'm a Fishbase collaborator. I'm now mainly interested in fish husbandry, fish health, native marines and fish photography.

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