We had a terrible outbreak of Cyano. Was able to remove it via a full blackout (not just lights, literally black garbage bags taped around it) for 3-5 days until none is seen when you look in. Water change as needed (watch parameters as it begins to die off). It looked like a brand new tank after 5 days and it was insane. We were far more careful and watched parameters and WC schedule far closer and it has never returned since (over 6 months ago).