In aquaculture we stock our fish at elevated densities relative to the natural environment, causing the natural processes to happen faster than they would in the wild. Fish and filter bacteria breathe, releasing carbon dioxide into the water which forms carbonic acid. Also, during the aerobic breakdown of biological wastes, such as the conversion of ammonia to nitrate in a biofilter, hydrogen ions are released and accumulate. This occurs as the carbonates and bicarbonates that the hydrogen had been attached to are stripped off and used by the aerobic bacteria. Both of these processes cause the pH of the water to decrease.
The result is that the pH slowly slides downwards as the water becomes acidic. For freshwater aquaculture most species are comfortable in the pH range of 6.5 to 8.0. Left unchecked a decreasing pH can drop well below the 6.5 level, causing problems on various levels to the fish.
Low pH of the water causes the blood of the fish to become more acidic and therefore less able to bind with haemoglobin. As the pH drops below 6.0 an increasing proportion of the nitrite in the water is present as nitrous acid which is the more toxic form. Fish egg survival drops off rapidly when the pH drops below 6.5. In a recirculating system, the bacteria in the biofilter cannot function properly at pH levels below 6.0, reducing the rate at which toxic ammonia is converted to less toxic nitrate.
In order to stabilise the pH we add a buffer to the system. The buffer provides a supply of carbonates to attach to any free hydrogen ions when they appear. As a result, the hydrogen ions never get a chance to accumulate and drop the pH, and the pH level remains fairly stable.
Various buffers can be used with consideration being given to availability, price, aggressiveness, solubility and composition. Each buffer is a source of carbonate (or bicarbonate) that is attached to something else, and the something else may not be desirable in the system. For example, sodium bicarbonate is freely available, highly soluble, inexpensive and non-aggressive, but the sodium is a problem in aquaponic systems.
How much buffer to use is best established by trial and error in your system, in accordance with the level of feeding, your choice of buffer and the alkalinity level of your source water. The important rule is Go Slowly! It is far better to add small amounts of buffer daily than a large amount once weekly. As a guide, the amount you add should bring the pH up to around 8.0 (6.8 in aquaponics). If you raise the pH to above 8.5 the ammonia in the system becomes more toxic and should there be any hydrogen sulphide around it too becomes more toxic, so do not overdo it.
Remember, manage the water and the fish will look after themselves.
For more information about South Africa aquaculture, click here