my levels are ok ..temp is at 77.lights are on for only 7 hours...what can be the problem ..theres good flow.corals are doing really good..the only thing is my sand looks ugly..it green with brown red...i use r/o water
What type of water are you using? I had a terrible time with algae for a few months after moving my tank but it finally cleared up after a large number of RODI water changes. I also noticed that using dry fish food seemed to add to the problem so I switched to purely frozen food. I haven't had a similar outbreak since.
A 4" sand bed can be problematic. It's not deep enough to be de-nitrifying, but it is deep enough to contribute to water quality problems because stuff is getting trapped in it and it can't be kept clean. 1" to 2" is much better.