It's REALLY tight with the short legs
http://www.aquarium-supply.biz/produ...eblacklegs.jpg
I have the long ones below on my 20" powerquad (I don't think it says they are for all fixtures but they are on my light and work fine.)they extend like crazy becuase they are longer and have two mounting points for the screws. I would for sure get these ones and not the short ones.
http://www.aquarium-supply.biz/produ...smountlegs.jpg
Hopefully this helps you out.
Greg