Originally Posted by redlessi
make sure it is a tank that can hold water for fish, some reptile tanks are really thin and not designed to hold water.
This was the first thing that went through my head too. If the tank was intended for reptile use only, like a critter cage, then it will not be a good idea to use it as a fish tank. Could you take a picture of it? Also, measure the thickness of the glass, and inspect the silicone seal to make sure it still looks decent. It's very important to find out if it is even an actual fish tank first.
I would HIGHLY recommend staying away from using any kind of harsh chemicals, even bleach to clean the tank. All you will need is to rinse the tank thoroughly, and maybe give it a scrub with a clean brush or rag. Use hot water if you wish, and a small amount of vinegar would be okay too.