aquaworld

Before attempting to change the pH of aquarium water, you should ask yourself if it is really necessary to do so. The pH range quoted for a given species may be based on its native waters. Although it might be desirable to mimic these conditions to some extent, the fish may be quite capable of thriving at a slightly different pH. It is also worth considering that many fish acquired in the hobby may have been aquarium bred for many generations and already become accustomed to water conditions quite different to their natural habitat. Providing a pH is usually more important than the exact value, as long as extremes are avoided.
Increasing ph
1. Aerate the water, driving off the carbon dioxide (CO2)
2.Filter over coral or limestone
3.Add rocks containing limestone to the tank or use a coral sand substrate
4.Use a commercial alkaline buffer.
Lowering pH
1Filtering water over peat
2.Add bogwood to the tank
3.Inject carbon dioxide CO2
4.Use a commercial acid buffer
5.Water changes with softened water or RO (Reverse Osmosis) water.
Attempting to lower the pH of well-buffered water with commercial chemicals or acid solutions
is likely to result in a losing battle, as the buffer system causes the pH to restabilise at its original value.The resultant pH swings are likely to be harmful to fish. The solution is reduce the buffering capacity or carbonate hardness (KH) first.
Apart from the effect of pH itself, there are important effects on the toxicity of ammonia and nitrite with changing pH. Therefore you should be particularly wary of attempting to change pH when either of these waste products is detectable - in particular, during the cycle. It is safer to let the cycle finish before attempting to adjust pH - it may settle at a different value once the cycle is complete in any case.