Cyano? Help please.

[carriej]

So this is my first real issue with any kind of algae, and the tank has been running well over a year so I guess I have been lucky, but this stuff sucks!

I have had conflicting reports from others - Some believe it is cyano, some say diatoms. It's slimy, stringy, and it generally disappears at night... Like totally gone at night. In the morning it's in full bloom. I tried removing it + doing a black out for three days... It came back every day regardless of no light (tank is in the basement - so no residual light). Now it's worse than ever.. Coating mostly my sandbed. The bottom of some rocks are affected but it's mostly the sand.