is cyano photosynthetic?

I have a couple little patches starting.I do my water changes with ro/di(store bought) and i also have cheato in my tank thats growing well.So if it is photosynthetic,i'd like to do a black out for a couple days,i'm wondering if that can get a grip on the cyano,without hurting the cheato(plus theres a couple small macros in there)I know the corals will be fine,just don't want to kill my algea.

 

Yes it is... But it will likely come back after a black out. You have to get to the root of the problem

 

What are some problems that can cause it? I started the tank seven months ago and i started with tap,but have switched to ro some time ago for water changes plus i skim as heavy as my modded seaclone will skim,and have added cheato to the tank (i have no sump).There is a reefcleaners massive cuc in there(for a 20 gallon) and i can't find the problem.I do test the basics and the params are good with 20ppm nitrates(i'm trying to get that down with a series of water changes.)

 

If you can get a hold of a TDS meter, check your RO water. Usually RO water will still have about 50-80 ppm of TDS. RO/DI will be near 0ppm. When I had a recent (last year) cyano outbreak, my bulbs were nearing the end of life. As soon as I changed them, the cyano died off. The first time I experienced cyano, my RO/DI filters needed to be replaced, they were giving water with 100ppm TDS, which resulted in high PO4 levels in my tank. You may also want to test for phosphates and silicates as those (and nitrates) are the main food source for cyano, and algae as a whole.
Keep in mind your water changes really won't dent your nitrate levels. Since you have a 20G tank, let's take this example. If you do a 10% water change, you will effectively remove 10% nitrates. That means your nitrate level will drop by 2 (assuming 20ppm), which brings you down to 18ppm. By time you finish your water change, your nitrifying cycle will replace that 2ppm that you removed 10 mins prior. The only time you will see a drastic nitrate reduction is when you do a heavy (50% or more) change. 20ppm nitrate is safe for a tank. We try to get it to 0, but it is *VERY* hard to get it to 0. My tanks are always about 10-20ppm.

Add:
The use of tap water in the beginning may be what the cyano is using right now and it may starve itself out after it uses up the silicates.
What's your circulation like? How much flow is in your tank?

 

+1 tigeremike74...

Algae problems are usually caused by aquarium water with excess nutrients like phosphate, nitrates etc. You can try and siphon out what you can to remove the algae in the short term. In the long term you may have to examine how you care for your tank. There are some commercial dry foods also contain excess nutrients and other undesirable things like nitrates, phosphates etc so I strain/rinse my fresh/frozen foods before feeding my tank. You may want to consider feeding your fish less often. Also using RO/DI water and a high quality salt with little if any NO3, phosphates etc like Tropic Marin will really help too. Adding a phosphate remover and increasing flow in your tank may help too.