Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If the GH is at least 3 degrees, then it is not likely Ca, though it could be.
Is this tap water? Or is it RO?
If it is tap water you can probably read a water quality report and get some idea what the GH, Ca and Mg levels are. As long as these are not zero, then it is rare for Ca deficiency to show up.
If you are using RO water, and not adding GH booster, then perhaps it is Ca deficiency. Fish food does not have much Ca.