I dont know about the brand you're using, but I use Prime. It can be used at several times the recommended concentration without becoming harmful, so accidentally putting one too many drops in a gallon won't hurt anything. This is mostly for when it's being used in aquariums in case there's a major toxicity problem that needs to be taken care of immediately, but it gives you a nice buffer zone when you're using it to purify drinking water for your other reptiles. I imagine other brands might be similar in that aspect.