I ran out of my usual 5/16" I.D. racking tube and all I had handy was some 3/8". It was a loose fit so I secured it with a hose clamp. Turns out if you over or under-tighten the clamp, you get some free aeration due to the venturi effect.
This is unlikely to get you to ideal O2 levels, but if you're not doing anything, it's cheap and easy.