A few drops of bleach works wonders... I was h my tanks out with bleach and there is aways a small residue left and that keeps the slime away I don;t have to clean more then once a week then and also keeping the tank out of the direct sun will works wonders also. If possible that is.. I keep mine inside there shelters so little if any sun gets to them heps a bunch, But One tank is out in the sun some.. But the Bleach in the water works fine..