It could be the phosphates. I have read that they should be below 0.2 ppm in reef tanks. However, some test kits won't read that low. My API test kit won't. It goes from 0.5 to 0.
I suggest you do the following if you aren't already:
- Continue to remove the algae from the tank.
- Make sure you are doing water changes. 10% weekly or 20% every two weeks.
- Use RO/DI water for water changes and top offs. Tap water often contains phosphates. If you don't have a RO/DI filter, I highly recommend getting one. Use Google to find a good price. If you are unable to get one right now, you can get RO water from machines at grocery stores and even Walmart for very little. Another option is distilled water.
- Feed less. If feeding daily, try feeding every other day and only feed what fish will consume in about 5 minutes.
- Run lights for shorter period. I believe 8-10 hrs is all that is required for most inverts.
- Replace old light bulbs. Old bulbs lose their good spectrum and can promote algae growth. I believe most fluorescent bulbs are good for about 12 months. MH bulbs even less.
If after doing the above you still have problems with algae and phosphate, you could try a Phosban Reactor or similar uplift media reactor. To use it, you put Phosban (or some other brand of granular ferric oxide media) inside the reactor and use a small power head to pump water from your sump or tank through the reactor. After a few days, your phosphate levels should be very low.
