Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If it is a very small leak you might use Vaseline.
I have several old canisters, and they are developing air leaks like you say. When I took apart the intake tubing, fittings and so on, then put them all together with a layer of petroleum jelly that was just thick enough to do the job.
This will not work on too large a leak, but is a cheap and easy first step.