Is that a lot of bubbles in the water? From an airstone? If so, that is probably keeping the CO2 level down too low. BBA seems to show up anytime there are problems with the CO2 level in the tank. Starting with DIY CO2 for a 20 gallon tank, you have an uphill battle keeping enough there, but if you use an airstone too, you will run low on CO2 for sure.
If the bubbles are CO2, it would help to find a way to get much smaller bubbles. A DIY needlewheel powerhead would help with that.