08-29-2009, 04:36 PM
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Romad, it would be worth knowing the KH of your tap water. Hardness (which you may know is distinct from but closely tied to pH) can be measured as GH (general hardness) and KH (carbonate hardness). It is the KH that we should know. KH acts as a buffer to avoid pH swings. But this depends uon the degree of KH. Water with no KH [my tap water here has zero KH and GH] will allow the pH to fluctuate according to other factors that a higher KH would buffer against, such as natural acidifying of water in an aquarium.
A drop of 7.6 to 6.0 in one week is very significant, and could be extremely stressful on the fish. While a partial water change to restore the pH seems like a good idea, it might not be; if there is something causing this sudden drop and it is still continuing, a fluctuating pH back and forth is almost certain to kill some fish or at the very least leave them weak and prone to other problems. Until the reason is identified, I would suggest allowing the pH to remain where it is in the aquarium. The effect on the fish will be no worse, as it is the movement that is stressful.
If you can provide the KH of your tap water and the aquarium water just for comparison, one of us should be able to suggest the remedy. Not that you're intending to do it, but on no account use pH altering chemicals, since these will not be effective long-term if there is a buffering agent.
One other thing, how long has this tank been running? And is this the first drop in pH, i.e., for the previous weeks (if more than a week since setup) has it always been at or close to 7.6?