As others have said, Prime is widely considered the best de-chlorinator. It's a great choice, especially for larger aquariums since it is so concentrated. I don't like it for my smaller aquariums because it's hard to dose out 1/10 of a capfull of the stuff. I use Zip Drops since you can use it by the drop. Whatever you choose, though, I recommend one that will treat both chlorine and chloramine. Not all on the market do.