Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Peat moss tends to remove positive ions from the water, like Ca++ and Mg++. Obviously, this is what we measure as GH.
Replacement needs to be with something that contains Ca and Mg.
Some peat does this more than others.
Peat adds H+ to the water.
Amazonia and many other substrates remove the carbonates from the water.
Without carbonates the pH can vary according to whatever else is in the tank. In this case, the H+ from the peat moss, and the organic acids will lower the pH, and it may get below 6.
I would add some minerals to the filter, and adjust the dosage until you have the right amount in the filter. You can add some of the same material to the substrate, but if you add too much it is impossible to remove just the mineral source.
Oyster shell grit (sold for caged birds like Budgies)
Limestone based fine gravel or sand
Are all possible materials for this purpose.