Understand MoeK dubbed it the scourge of death and rightfully so years ago.
The longer you wait the worse the fight will be. If you are at the point you can see them forming in the water column at lights on then they are rooted pretty deep. if within 10minutes the coat everything in the tank you are where I was with them and its no easy task.
If you beat them back and they return after you stopped fighting them, what ever you did, do it harder and faster. Get them on the run and keep them on the run. You say the returned what time frame are we talking? weeks, months or years. if its week or a month or two you didn't beat them all the way back. So go back to what worked plus lights out to step up your game.
If you get them in control and they return, as soon as you see a hint of them blast them.