Aqueon® Submersible Aquarium Heater ..

Top Fin® Digital Aquarium Thermometer.
Photo provided by Flickr
Unless the water is flowing trough housing there's no problem. If you switch off filter (or something will stuck in pipe, might be thousand reasons) and there's no water flow, the external thermostat don't know about water temperature inside housing (because sensor is in the tank) and will heat until the water will boil inside. Housing, seals, heater etc might survive that accident but if I can I wanted to prevent stuff like that. For that reason there's two thermostats one (in heater) set up slightly more than normal temperature in fish tank (my is 26-27C, lets name it security thermostat), and the second one actual thermostat for aquarium temperature (external one).
13 items - DHgate is the best place to make a comparison for thermostat for fish tank. Compare prices on thermostat for fish tank to find great deals and save big.
Photo provided by Flickr
Most aquarium heaters and some aquarium chillers come with built-inthermostats, so you have a control right on the heater or chiller toset the thermostat to maintain the desired temperature. On handingheaters, this is usually on the top of the heater in the form of aflat knob. For submersible heaters, this is usually a small knob ornipple sticking out of the end of the heater next to the cord. Thisknob or dial can be turned one way to increase the temperature settingfor your fish tank, and the other to lower it. Buy High Quality Digital LED Temperature Controller Thermostat for Fish Tank Aquarium Reptile 110/220V at Wish - Shopping Made Fun.
Photo provided by Flickr53 items - DHgate is the best place to make a comparison for fish tank thermostat. Compare prices on fish tank thermostat to find great deals and save big.
Photo provided by FlickrMar 9, 2017 - This amazing fish tank thermostat allows you to control the water temperature for the comfort of your lovely fishes or other aquatic preference.
Photo provided by Flickr

Just because you can set the thermostat on your heater, you still need a thermometer to measure the temperature! Unfortunately, heaters can fail. They can get unplugged or even get stuck on heat. That’s bad! Avoid using an oversized heater. For aquariums larger than 60 gallons, it’s good to use two smaller heaters set to the same temperature. If one dies, usually the other keeps the temperature safe. For most home aquariums, 3-5 watts per gallon is the simple rule of thumb. For example: A 29 gallon tank would need a 100 – 150 watt heater.
Learn a great habit. Check the temperature every time you feed the fish. Get a nice thermometer that you don’t want to hide. Digital thermometers are easy to read. The ones that store the minimum and maximum temperature are really useful, that way you can see how far the temperature varies each day or each week. There are also aquarium heaters and aquarium chillers that do notcome with their own thermostat. These heaters and chillers willrequire you to purchase a separate thermostat unit to control theheater or chiller. These are particularly handy when you have a tankin a situation where it is likely to be too cool during winter months,but too warm during the hot summer. With an independent thermostatlike this, you can often control both a heater and a chiller with thesame thermostat and maintain your aquarium temperature in the correcttemperature tange for your fish that way; using the chiller or theheater as appropriate, but not having the two compete with eachother.Some other preset aquarium heaters claim that are set to thecorrect temperature t the factory and need no adjustment. These shouldbe viewed with even greater scepticism, and probably should beavoided. Since these heaters have no way to adjust the temperature,you cannot set the temperature correctly for your fish that preferwarmer or cooler water, and you have no option to elevate thetemperature in the tank in case of . An aquarium heater without anadjustable thermostat also means that the heater cannot be adjusted toaccount for differences in power stability or for age of the heaterand components.To achieve this in an aquarium you must install a heater in the corner of your tank. The best way to save money on heater is to buy a heater and thermostat combined so that water temperature is maintained and does not exceed 24 degrees. If you want to put just gold fish in your tank then you do not require a heater as these require a temperature of 17 degrees because these are not tropical fish. Most heaters come in the form of a vertical tube with rubber suckers one below the other. These stick to the inside of the glass tank dispersed in water.