due to many's poor experience including my own, I'd suggest their deluxe visitherms instead. I sent mine back when it didn't shut off automatically when water level went down & it smoked me out/melted the holder!
cost me $9 so any kensfish savings I had went out the door!
I haven't had any issues (burning, failures) with any of my brand heaters new and old although I am partial to Visi-therm because the dial indicator is easy to see even from a few feet away and the big knurled knob is easy to grip turn. The key is never allow any heater to be exposed outside water while plugged in regardless of features. Either set it deep enough not to be affected by water-changes or get in the habit of unplugging 10 minutes before starting a water change.
As far as size, just follow the manufactures recommended size chart and try and place the heater near the outflow of your filter for good heat distribution. Doing this you should only need 1 heater which is what I use now becasue Im always in the habit almost everyday of looking at tank temp or touching the water, and my house temp is always between 72 and 80 and have never had a problem even when I have forgotten to plug a heater back in, I always catch it in time.
