]Bavass is right...two of the most common nutrients that cause brown or red slime algae/bacteria are Phosphates and Nitrates. Check your system's wate, and also your tap water or r/o water. If your fresh water is reading 0ppm, then it's in your system. There are many chemicals and supplements out there that can help with this problem. Unfortunately for me, I have yet to find one that works for me. I'm in the early stages of building a Denitrator
. I have been fighting Red Slime for 2 years, and I have run out of options. Hopefully, your issue isn't as severe as mine.