I got a 65 gallon tank on craigslist a long time ago and never got the chance to actually use it, it's just been sitting in storage. After we're finished redecorating the room it's going in I planned on just filling it up for the first time in the house, then I realized that wouldn't be a good idea in case it leaks.

I don't think it would because the guy I got it from was really nice and gave me the tank and everything for free (so he would have told me if there were problems with it) but I don't want to risk it. And I guess maybe the sealant could have gotten weak in places since I got it or something like that.

Anyway, I'm going to test it outside first but how long do I actually leave it filled with water before determining that it doesn't leak? Is a day sufficient enough?

I'd probably give it a few days, just to make sure.

Also, make sure you have it on level ground. If the ground isn't level that could cause damage to the tank once it's filled with water.