Algae is caused by excess nurtients. Wee little plants. UV
sterilizer won't help at all because algae attaches to a surface and grows. It doesn't float around the tank like some bacteria do.
Having no fish, I'm wondering if your feeding the CUC in your tank?
Also, what type of filtration system do you have? HOB
(hang on back) filter, Power filter, sump?
Any nutrients you put in the tank (which will be reduced because of the new RO
system you have), including any food, eventually has to come out. Either cleaned with a filter, skimmed with a skimmer, converted (to nitrogen) with bacteria or chemically via resins.