I have a Aqua Medic 250 watt over my 20 X 20 X 9 clam tank and am very happy with the choice. I have a fan on the same timer as the light to keep the temperature down. The thing I notice most is my evaporation is very high (10+ gallons per week).
I added these pictures showing the light & fan and the aquascaping to my tank thread showing what a 9 inch deep tank looks like:
http://www.canreef.com/vbulletin/sho...4&postcount=97