Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
What is the GH in the water?
I would raise the KH in 2 steps.
Add 1 teaspoon of baking soda per 30 gallons, let it circulate and test the next day.
If something in the tank is removing carbonates then the KH may hit zero again. This would be a slightly different problem.
If the tap water has zero dKH, then adding the baking soda ought to stay stable overnight. The day after that, add some more. This gives the fish and livestock a few days to acclimate to the rising levels. Then, each time you prepare water for a water change, bring the KH up to 4 degrees before you add the water to the tank.
Continue to monitor it, when you are adding CO2 there may be an interaction, and you might need to add a bit more between water changes.
Add coral sand, oyster shell grit, limestone sand or other material to the filter. These will dissolve over time and help maintain the KH in the tank. They do add calcium and magnesium, too, raising the GH. You would still need to prep the new water to the right levels.