I read from this book published by Barron's and written by Barlett and Barlett, who are supposed to be one of the leading experts on this subject, and it saids, a minimum for an adult is 50 gal. Someone on here saids that they excrete a lot of waste into the water because they eat a lot, and you should clean the tank and change the water often like once a week without a filtering system. I think the main reason for some people frog deficating so much is that theirs are fed too often. I think you only need to feed it once a week. They are cold blooded; they don't need to eat like us and dogs and cats. Despite this, They will eat everyday due to a survival mechanism; it's an insecurity of feast and famine, and they will gorge themselves if the opportunities arrise, as a result. Plus, I read somewhere that if it's actually way to big, although larger than 50 gallon is probably better, the frog can feel insecure. Frogs are more like sit and wait predators, but they will crept up toward the preys ( especially bullfrogs from far away ) from quite a distance if they spot them.