What baffles me most is 1 little 25 litre tank only stocked with shrimps and snails never fertilezed standing for a few years periods heated and periods not heated, it stands beside a window and has only natural light. It never even grew a spot of BBA, if i throw BBA infested plants in there they come out clean weeks later.
I have similar experience as yours. I have a one gal planted shrimp bowl by the window (in the avatar) that has no heater, no filter, and no artificial light, just receiving 4 hour direct sunlight daily. The plants are lush with no trace of BBA from day one I set it up. I transferred some BBA infested plants from my big fish tank, and the BBA are gone in few weeks. I do not fertilize or do water change, except I top it with dirty water from my big fish tank regularly. Nitrate is always detected near zero. Occasionally I do negative WC by replacing a few ounze of clean water in the bowl with dirty water from my big fish tank.
The shrimp are rarely fed, and live on dead plant tissues, algae that I can't see, and biofilm. There are snails and variety of tinny critters crawling around. The bowl is totally free of algae of any sort that I can see. Is it UV from direct sunlight or insignificant bio load that keep BBA out.