Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
1) If the KH is really low, then the pH can vary a lot. It used to be thought that the KH needed to be about 3 German degrees of hardness to buffer the pH, but many people are reporting stable conditions with just 1 or 2 degrees KH.
2) Unstable pH does not seem to be a problem for fish when it follows a day/night sort of cycle because of CO2. (Plants use up the CO2 in the day, pH rises. More CO2 enters the tank at night, pH drops)
3) If the pH is variable because of something else, then it is probably the 'something else' that is causing problems for the fish. Such as suddenly changing mineral levels with water changes. When doing water changes match the GH, KH and TDS. Do not worry about the pH. For Rams, I would filter the new water through peat moss. (mine bred several years ago, so I think I was doing something right)
4) Low pH (under 7.0) usually means that any ammonia is present in the ammonium form, and is less toxic to the fish. Plants can use either form.
5) pH under 6.5 is very difficult for the N-cycle bacteria. They thrive with the mineral levels usually associated with a pH in the mid 7s, and can handle pH 6.5 to 8 pretty well. At pH 5, I see serious problems with the bacteria part of the bio filter. This need not be a problem in a planted tank.
6) I am not sure which forms of algae are favored at that pH, but I am sure some form is adapted to thrive under those conditions.
How I would handle this:
Check that the GH and KH suit the fish. Ignore the pH unless you find there are some problems that you think need to be addressed. If you want to raise the pH, you can add some carbonate or bicarbonate (baking soda is one source) to raise the KH, and this will raise the pH.
Dosing: 1 teaspoon baking soda added to a standard 29 gallon tank with a canister filter (assume 30 gallons) raised the KH from 0 to 2 degrees. The pH in this tank tested at the bottom of the scale (6.0) and came up to 6.2.
If you want to use another form of carbonates I am sure there are recipes on line. Some forms may also raise the GH (Calcium carbonate) or might be hard to dissolve. I have not used these.