[you will probablly have to go to a lfs to have that tested....is the red slime growing all over, or just in certain spots? Dead spots with no current can occur even when you think you have enough water flow. A lot of time it takes some adjusting of your powerheads to get them just right. You may also want to try "chemi-clean" or "red slime remover."
Where are you getting your water? Is it RODI?]