They make speciality clamps for tubing/hose that small just like regular hose clamps like on a radiator hose for example.

Sometimes you can find them at specialty electrical supply places.

I've never put one in a tank like that just pointing out another idea.

Also I've seen wire clamps that basically go around a tube and come up on each end to form a "V" you squeeze to put it on. Everyone has seen these tiny ones, but I've seen a few that have a flat spot on the other side to keep them from biting in.