As far as tap water conditioners go I recommend using Prime. It's good to have around in a pinch if an emergency should arise. I've never seen any effect on corals or fish from using it but the only time I ever used it for any period was back when I started out and didn't have an RO unit and the tank was mostly SPS. I also like it because it's potent enough where you don't need that much of it, I think a cap handles 50gals.