Actually... My custom 17 gallon tank was a terrarium at one time. I got it for it's size and dimensions. Fish I don't know too well, but I did a lot of research on how to build tanks, because I had a really cool custom one in mind.
Here's what you do. Find out how thick the glass is on the tank and compare it to the thickness on a 20 gallon fish tank. I don't mean a rough estimate... I mean exact measurement. If it's the same, or thicker than the aquarium... You're in business.
Check the inside corners of the tank. It will either have a caulking bead of silicone (like you see on the joints of regular fish tanks) or it won't. If it does, you're good to go. If it doesn't, you will have to put it there. All you have to do is buy some aquarium grade silicone at your local hardware store and run a smooth, bubble-free bead of it along the inside corners of the tank. You should be able to find instructions for this online, or at a good hardware store. You have to let it cure for a week though, otherwise you're gambling with the lives of you fish. And you have to use aquarium silicone... anything else would be toxic to your fish... All in all it should cost you about $7.00 to do.
The hard part is putting a solid (Read: non-screen) hood and lighting on it... This will prevent evaporatin. If you custom make the hood out of clear Lucite it should cost you no more than $10. It's a bit of a project, but it's fun to try your hand at things you've never done before... Just take your time and be careful. As for cleaning the tank... If it has had any other animals in it, you should scrub the heck out of it... Just don't use cleanser, soap or bleach. This will be instant death to fish.
BTW: My tank has been holding water for 8 months with no leaks and no bowing of the glass... All because I chose the right one and took my time reinforcing it like a fish tank.