We used our 125g years ago. It housed our RTB for two years prior. Some people say that the seals/silicone will dry out etc. We had no problems though.
Everything carries a risk. You just have to weigh which and what is too much risk in your scheme of things.
If it is 'technically' a reptile tank, then no, I wouldn't trust it as they are made of a thinner weight glass than a real aquarium.
If it is actually an old aquarium and has been cleaned VERY well, go for it (after testing to make sure you don't get any funky reading as Brendan said).