Review on Tetra HT Submersible Aquarium Heater: Efficiently Regulate Temperature with Electronic Thermostat by Dana Smith

I like these. I have maintained tropical aquariums for 45 years and temperature control is a breeze. I just put them in the aquarium, give them time to acclimate and plug them into the mains. Nothing needs to be adjusted. Most tropical fish can tolerate temperatures in the 8-10 degree Fahrenheit range and these factory set heaters are within that range. All my fish are perfectly healthy and spawn regularly. Here's another tip: instead of buying 1 heater, buy 2 at half the power you need and put one on each side of the tank. This means that cooking or freezing fish is practically impossible in the event of a malfunction. The indicator lights will let you know if you have a problem, and if one doesn't light up, the other heater may be hot enough to keep your fish alive until you replace the faulty one. Or if one gets stuck, it won't be strong enough to cook the fish before you know there's a problem. I have had instances where the heaters failed in both directions killing the fish. Unfortunately, sooner or later ALL heaters, even the most expensive ones, fail. If nothing else, eventually the cable will fail. This plan is the best solution I've found. Don't panic if they're not perfectly in sync and one of them is doing most of the work. If your aquariums are in your air-conditioned home, I've found that you have nothing to worry about. Much luck!