With all the power outages due to the storms, I figured I would post a link to this. They work great. I have 3 on tanks at work and one for my home tank. I have run them for 6 hours on the battery and haven't had a problem.
http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+3669+8120&pcatid=8120
http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+3669+8120&pcatid=8120