04-26-2010, 12:23 PM
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There are two different but connected issues in this thread.
First, tap water is pH 7 and you want the tank higher for rift lake cichlids, say pH 8 or above. The easiest, safest and less costly way to achieve this is with calcareous material as DCR mentioned. I have previously kept these fish in pH 8 with tap water of 6 by using dolomite as the substrate. In your case, as the tap water is already at 7 and may contain some hardness, you could have a quantity of dolomite in the filter that would raise the pH and corresponding hardness from 7 to 8 easily; some initial experimenting to find the amount necessary would be required over a couple of weeks, but it doesn't take much. This method allows you to have the substrate of your choice, say buff sand for authenticity. There is also the rift lake cichlid substrate sand available that contains the mineral content to raise pH and hardness. Dolomite seems difficult to find some places, and I have heard tat crushed coral works the same way.
As for partial water changes, which I thought was your original question, I would do nothing. Once the tank is established with a stable hardness and pH of 8, a partial water change of 50% with tap water at pH 7 would have minimal impact. The stability of the tank's biological system works to buffer new water, especially in your case with the higher mineral (calcium and magnesium) from the dolomite.
This works both ways; in my present setups I have tanks with a pH of 5 and 6, zero hardness; and tap water is 7. When I do the 50% pwc, the tank pH rises maybe .2 or .4 at most which is insignificant.