Red Algae?

[ReefCresteds]

OK-I have a red algae question-hope somebody has some thoughts on what could be causing this. First of all here's my setup:
I have 3 reef tanks all running into one large sump. The tanks are a 300 gal, 185 gal, and a 120 gal-running w/ about 100 gal in the sump. so i have about a 700 gal setup. Everything has been growing really well. The setup has been running for about a year and a half. My nitrates are at 0 and I don't have an algae problem in any of my tanks except recently (last 2 months) I have been getting red slime algae growing on the substrate in my 120 gal. I don't have it in any of the other tanks. The 120 has a lot of light on it-2 400W halides w/ VHOs(it houses SPS and clams). I have one tang in here-a yellow tang.
Any thoughts -what should I do to get the red alge under control in this tank? I am siphoning it out almost everyday off the gravel.

Thanks for your thoughts.

[on3ofak1nd]

By red algae you mean Cyanobacteria? If so then what steps have you taken to reduce the amount of dissolved organics in your system?

It is strange that it is only occuring in one tank. Would it be possible to isolate this tank from the system until you can eliminate the problem?

Are you skimming heavily?

Do you run carbon?

Do you have any other types of mechanical filtration?

Do you have a DSB?

[ReefCresteds]

Yes-cyano. i have reduced the feeding in this tank. I think that is what started the problem initially-overfeeding in that one tank- since there are only a few fish in there.
yes I skim heavily.
no-i don't run carbon
no-other mech filtration.
no DSB-the sandbed is only about 1-2" deep.

thanks