Weird Algae problem

[Mich3417]

I have had a reef tank for the past 7-8 years.... I have a 200 gallon tank with a subsancial (plenum) sand bottom. All my readings are with in normal limits except my nitrates are around 5ppm which is normal for this tank (more fish then I should have). In the morning my sand is clean and an hour after the lights go on, an algae ?(brown) appears on the top. About an hour after the lights go off it dissapears. I thought maybe this could be some kind of cyano bateria and I tried treated the tank with chemiclean.. No difference. No sign of this algae? on the rocks, corals or side of tank...any idea what this could be? It covers the sand by about 1/16 of an inch....

Thanks..Mike

 

[billsreef]

Most likely diatoms.

 

[BeanAnimal]

What are you using as top off and change water?

 

[Mich3417]

RO-DI, automatic fill. I do a 5 gallon water change a week. No algae at all on the rocks or corals. Tank is crystal clear... small amount of green algae forms on glass which is usual, I clean it maybe twice a week. But that dark algae? is on a good portion of the sand durring the day.

 

[BeanAnimal]

Still sounds like diatoms... is there any other source? Is this tank at home or in your office?

 

[conefree]

When you say automatic fill, do you mean the RO/DI unit output goes directly into your sump? If so, you could have a TDS creep issue with your RO membrane.