Yes, it works. I had mine set to 90f... it would go from 1/4" up the pots down to barely covering the heater in just a few days. I ran no fan... This is when I first set it up... After they had been in there for a while the pets looked great. I even divided the D. ordensis approx 15 times and traded/gave away the portions... After I took the tank down and moved the plants to the (old) greenhouse, all of them died once winter came around. Lesson learned. Here is the picture, hope it helps!
http://aaadnedarn.home.comcast.net/~aaadnedarn/P1010248.JPG (sorry the picture is big.. it's old and was already online so I didn't resize it..)
My guess is that was a gallon or so... How big a tank are you going to try and heat? higher wattage could provide more heat with faster water evaporation... Honestly, I think mine was a 150w....
Good luck!