hound and keeko are right

ya might be able to open one of the short sides and close the long opening with something but even after that you'd have tons of work to modify it and I don't even know if you'd get enough to smoke

Is there a particular reason you were thinking about using a fishtank...a garbage can would probably work alright with enough modifications, I've seen people grow some reasonable mj in those things but the fishtank is probably too small