Do a search of the site under "red slime algae".
There are several good threads discussing treatment.
I'm currently having red slime problems myself, but I'm a little reluctant to go the chemical route. I thinks it's better to determine the root cause and correct it.
When you say all your tests are normal, have you tested for phosphate in your water? Are you using tap or RO? I'm currently using tap and I THINK that's my problem.
Bluto