I would NOT recommend that slim Hydor heater. You should try to order an 25w-50w ADJUSTABLE heater online, you should be able to get one for about $12 maybe less.
It's just nice to be able to raise/lower the temperature a few degrees if sickness occurs.
I was playing with the hydor heater yesterday & it took about 4-6 hours for the temperature to go from 74 to 82. I just came home and now it's 88-90 degrees. There's nothing I can do to lower the temperature other than unplugging the stupid thing. This is one reason I never bothered using it.
If you're going to buy a couple filters, go with the Tom version of the Fluval. It's looks like it's made a little better even though it's the same design. I like the color contrast of the media.
Funny KFry mentioned the Tom because I thought it looked similar to the Fluval.