Do you have soil substrate? I had nothing but problems with soil substrate, bga was a continual problem and I finally had to dismantle the tank. Anything decaying will also cause it (plants etc.)

You could change your drop checker fluid to be sure it's correct.

If you remove co2 to see what happens to the fish, I would keep the light off as well, this might help get rid of your bga. Keeping the light on while removing co2 could cause more major algae problems.

If your tap water has 0 GH and KH you might be ok replacing evaporated water that often, but keep in mind that anything in your tap water will be building up to much higher levels with that evaporation happening. If you can, you should only replace evaporation with distilled water.