I've seen BBA grow in low-flow and very high-flow areas, so I don't know if it's ever been proven that not enough flow causes BBA. Personally I had a 72g setup for years with just a Eheim 2236 (less flow than a 2217) and didn't have any bba or other problems. What is your lighting schedule and co2 levels. If you can I would increase co2, reduce lighting duration and stay on top of your filter cleaning possibily adding Purigen or other organic remover to your filter media.I am looking to increase the water flow in my aquarium in an attempt to eliminate bba. have an Eheim 2217, a HOT magnum filter and the Hagen elite mini (niko's DIY co2 reactor). How many powerheads should I have in a 55 gallon aquarium and what would be the ideal flow rate?