In an earlier post about emergency preparedness, I mentioned the use of controllers. One such controller is for your aquarium heater(s). Though most current aquarium heaters are extremely reliable with respect to a stuck thermostat, the thought of having one get stuck in the "on" position while you're away all day is quite unpleasant. For piece of mind, it's a good idea to use a temperature controller that will shut the heater off in the event of such a malfunction. If you use digital heaters, like Hagen's Fluval E series, which have a built-in, easy to read thermometer, remember that a temp controller shuts off the power to the heater so the heater's display will also cease functioning.
There are many controllers on the market, and you can spend a lot or spend a little. Many aquarists employ multiple heaters in large tanks ( >90 gallons). Some folks use multiple heaters on small tanks too, just to be safe. I don't. My largest tank is 75g, and I'm perfectly comfortable using a single heater. There are dual-controllers available that can control two heaters if you're a multiple heater user.
A heater that fails in the "off" position generally presents less risk because fish succumb much quicker in higher heat. Dissolved oxygen levels are lower in warm water, meaning if you have a heater that malfunctions in the "on" position, your fish not only have high temps to worry about but oxygen may deplete quicker also.
I've tested the Inkbird controller like the one pictured above, model C206. It worked fine, was simple to set up, and is quite economical at ~$20 each. Set your low and high temps (C206 will display C or F), plug your heater into it, then let the controller handle the rest. Don't worry about using it with big heaters. It's rated to 1100W. The digital display is easy to read and it displays the current water temperature concurrently with your high and low temp settings. Also, the temperature probe is extra long at 4 feet.
I see only two downsides to this controller. The least problematic is that temps can only be set in 1 degree increments. The bigger issue is that this model Inkbird plugs directly into an outlet, which for most aquarists make it a bit impractical. Most US electrical outlets are usually low on the wall rather than up near the top of the tank where the display would be easy to read. Sure, you can use an extension cord, but remember the heater plugs into the front of the Inkbird unit, which makes the placement of the controller a big awkward. However, these controllers aren't made specifically for aquariums, so the company wasn't inclined to take all of that into account when it designed them.
In any case, whichever temperature controller you decide upon is less important than the fact that you have one.
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