It was totally by accident, i baught a tank hoping to use it instead of my other, but found it's not water tight and leaks quite bad. so emptied it all out and noticed a little fish swimmin about, i hadn't noticed it before i'm guessing it was amongst the plants that were in there.
I had a closer look and found it was a puffer fish, but what do i do with it now? I can't keep it in my community tank, and can't keep it in the other tank because it leaks really bad. so now it's in a tiny breeding tank floating in my community tank. What do i do with it? Any ideas?

Can you post a photo? That way, we'll be able to ID it.

Did the plants come with the tank? If so, were they submerged in transit?

It a figure 8 puffer.
Yeah the plant's were in it, i wasn't planning on using them, i just wanted the tank. there was about an inch of water that the plants were laid in, i didn't think there was enough for a fish to swim about in.
I rang the guy i got the tank from, and asked if he wanted it back, and he said he doesn't, he didn't realise it was still in there.