Algae problems

[Les Chainey]

I have a 48 inch x 18 inch x 12 inch marine tank containing 1 Tang, 2 Domino Damsels and 1 yellow tail. Filtration is by 2 undergravel filters aided by powerheads, a Fluval 204 containing carbon and clearwater, a UV and protein skimmer. Lighting is by 1 38w blue moon tube lit permanently and 3 40w powerglo's lit for 12 hours a day. The tank was set up 8 months ago and all was fine until I introduced a Anemone. 2 weeks later it died closely followed by the macro algae Caulerpa. Since then I have severe slime algae problems. Black, red and brown slime algae covered everything, green algae grew on the glass and bubbles are stuck on the rocks. I have tried 20% weekly water changes with RO water but the algae comes back. 4 days ago, in desperation, I scrubbed the rocks, cleaned the gravel and all equipment and changed 90% of the water. 2 days later spots of black slime algae appear so I siphoned them off. Today more spots of algae appeared on the gravel, the bubbles appear on the rocks and the rocks are becoming covered in brown stuff. Temp is 80 deg F, Nitrate 5 mg/l, S.G 1.021, Ammonia 0 mg/l and Phosphate 1.0 mg/l. Is there any hope of a cure or do I turn to tropical fishkeeeping ?