Chaetomorpha, as most have said, isnt the best one for the display tank, most suited to refugia, mainly because it will encourage detritus and biofilms to form in the brillo pad-like mass. To keep growth up, rinsing it out once a week, when you trim, is the best course of action. Hard to do that in a display, plus you run the problem of having fragments form satellite colonies in the tank.
There are quite a few good display macros though and they will help a little in reducing nitrates over the long term.. Sargassum sp., red macroalges - Halymenia, Bortryocladia, Ochtodes, etc., and the green calcerous Udotea (mermaids' fan), Halimeda (cactus bush) and Penicillus (shaving brush) are all good options. Beware Dictyota sp. and the Caulerpas, which will both be too aggressive growers for the display.
Plumbing a small fuge isnt too difficult. You really might want to consider it.
>Sarah