Just bought a nano skimmer for my 20g QT

[tim the toolman]

They are actually not that bad. They really do a job in keeping the surface clean and you shuldnt have to much to worry about as far as the water line fluctuating. If you set the neck high enough you can easily get away with about a one inch drop in water line without any issues since it just floats and bobs.
Ran one for over a year on my 25g office tank hooked od to a canister and i loved it