They look like a cheapo version of visi-therm stealths. The claim to be "fully submersible and indestructible." Googling around I did not find a review but they are made by penn-plax which I've never had problems with. I was just wondering if anybody has used them. At $14.99 for a 250watt I figured it's worth a shot but wanted to see what other people had to say about them before I ordered a whole bunch.
http://kensfish.com/guardianheaters.html
http://kensfish.com/guardianheaters.html