20 tall....go big or go home!!LOL first off, i like the concept. very cool. how much water are we talking about? 50/50? 70/30? as far as limestone, i can't be positive about safe for the frog. i would imagine that a majority of limestones are pretty much alike, as far as how they affect water chemistry. they are all going to make it hard and alkaline. i would imagine that if it were a wc frog, that would be fine, but a cb, i wouldn't risk. but disregarding the frog for a moment, let's look at the other stuff you're gonna have in the tank. unless you use plants from a limestone stream, any plants you put in it are going to HATE the water. especially over time. most plants like a nice, neutral to soft and acidic water/soil combo. your soil mix may help to buffer it a bit. but probably not enough. but here's the biggie. water spots. you will be constantly cleaning water spots off after a few months (maybe less) you know those dishwasher liquid commercials with all the hazy glasses? there ya go