My tank is 28 gallons. I think it's well worth a shot, particularly if you have a h.o.t. filter you're not using--though even a brand new one is pretty cheap, if it makes a difference. If you're interested in the specific light I got (which is by no means a necessity, but it is a nice little unit and sits pretty flush on top of a h.o.t. filter box), shoot me an email. I'll tell you where I found it for $25. Email is uberlink at mac dot com.
The other advantage is that my ph has stayed much more constant and into the low 8's ever since I added this and started running the light at night-time. Previously, it tended to dip at night (as is very common) and then climb during the day, but never get much over 8.0 or so. Now it's pretty steady 8.2 or 8.3 all the time.