Stephan, The issue is overfeeding. 2-3X a day?? Drop the feeding down to once a day for a while. Even every other day. Trust me, you won't be starving the fish. And don't increase the amount simply because you're decreasing the number of times you feed. Keep in mind that the size of a fish's stomach is about the size of its eye, so very tiny. They don't need as much food as we think they do.
Also, the fish you named are much too big for 20g tanks. They'll outgrow them in no time.
And Kj-Lou, it's best not to add chemicals to reduce your pH. Get a nice piece of bogwood. That'll do the trick ... safely.