Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
The substrate is probably removing the bicarbonates from the water, so the KH and pH are dropping.
You can correct this by adding baking soda (1 tsp per 30 US gallons = 2dKH) or potassium bicarbonate once or twice a week. Just enough to bump the KH back into the proper range.
Nitrifying bacteria and some other organisms use the inorganic sources of carbon (including carbonates) so I would keep the KH around 3 degrees. Also, monitor the pH. If that is still a bit low, go ahead and increase the KH a bit more.
GH ought to suit the fish. Most aquarium plants are fine with a wide range of GH. This supplies calcium and magnesium to both fish and plants. Seachem Equilibrium is one the the GH boosters that has a good ratio of Ca:Mg, plus some potassium. I would dose this when doing a water change, then monitor it through the week. If it drops more than about 1 degree between water changes, then I would add some each time you dose trace minerals.
Here is how I prepare new water for a water change:
1) Make the GH suit the fish.
2) Make the KH equal the GH.
3) This ought to make the pH in the right range for the fish. Fish generally are not as picky about the pH as people used to think. The important parameters are the mineral and salt levels. GH, KH and (if you have a meter, TDS).
4) If you are keeping black water fish, filter the water through peat moss to add the organic acids these fish seem to need. You could add oak leaves, alder cones, or any of several species of wood to the tank to do the same thing.