I dont use them in tanks that small because my bettas always ended up getting sucked in. Mine is pretty powerful though, you could clearly see it effect in a ten gallon. I would use one if I could, but for smaller tanks entire water changes are easier. If it is really heavy planted I think you would be okay, but a filter would def help.
I have it on med in a 3.5 and it moved my moss ball, anachris, covered my javafern in sand and moved my sand around so I have a lovely hole in the tank. Saved me from redecorating >.<
If there are no fish, I would be to worried.