I've found that the blackout method works the best for killing off BGA/
cyano. Simply cover your tank so no light gets into it from the outside, turn off your tank lights, and let it sit for three to five days. Perform a 50-70% water change both before and after the blackout. During the blackout period do not feed the fish, fertilize the plants or run
CO2. Your fish and plants will do just fine so don't worry about that. If you can, try to run extra aeration (bubble stones, etc.) and keep the tank temp at 77F or lower. After three to five days remove the covers and you should find all of the BGA dead. This is the method Tom Barr recommends and I have used it several times with 100% success, and the best part is it doesn't cost a thing