Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Test the tap water.
GH is calcium and magnesium. Can come from tap water, but some fertilizer regimes suggest adding Epsom salt or GH booster. If your tap water already has enough calcium and magnesium, then do not add more with the fertilizer. This might be the reason it is so high in the tank.
KH at 40ppm is pretty low, and if it is dropping through the week then I could sure see the pH crashing several days after a water change. 40 ppm is just a little more than 2 German degrees of hardness. This level works in some tanks, not in others.
The nitrifying bacteria can take the carbon from carbonates, and there is an interaction between CO2 and carbonates. Some high CEC substrates remove the carbonates, too. I know Safe-T-Sorb and the ADA products do this. I do not know if any of the Flourite product line do.
Here is what I do to fix this:
Add baking soda or potassium bicarbonate to the tank in between water changes.
1 teaspoon of baking soda added to 30 gallons of water will raise the KH by 2 German degrees of hardness, or 36ppm.
Potassium bicarbonate would be dosed at 1 tsp per 48 gallons = 1 dKH.
I would dump these into the tank right in the flow of the filter, or else shake them in a jar of water to dissolve, then dump them in the tank (again, right into the flow from the filter). They dissolve easily.
You could also add some material to the filter in a nylon stocking.
Oyster shell grit (sold for caged birds)
The problem with any of these is that they will also raise the GH, and I do not think you want to do that.