Internal heater for aquariums is practically a resistance with thermostat introduced into a capsule of glass and is designed to increase the water temperature in the aquarium.
In some cases the internal heater can be completely useless (for fish that prefer cold water) or extremely useful if you have fish that prefer high temperatures. Internal heater is not expensive and ensures that there will be no sudden temperature drops. Moreover, it does not come into operation only when the temperature is one degree below the thermostat set.
For most fish, the recommended temperature is 24-26 degrees but you can different situations, for example discus prefers temperatures of 28 degrees. When you are buying a heater we recommend checking the glass of the internal heater because if it is cracked the internal heater must not be put into water.
If you wonder which internal heater is suitable for your aquarium I think it's ok to go Formula 1 Watt/liter. An internal heater having the heating power of 200W it is ok for an aquarium having a volume of 200 liters.
For internal heating I recommend you buy only trusted brands because the temperature can be precisely set, the operation of the internal heating is indicated and also it has all kinds of protections.
This type of heater does the exact job of the internal heater, the difference is that the external heater must be positioned outside the aquarium and you will not have so many equipments in your aquarium.
Now on the market you will find only one external heater and that is Hydor ETH External Thermal Heater. I had one but in my case after three weeks of operation Hydor ETH External Thermal Heater stopped to work anymore.
Besides it is very expensive the external heater must be cleaned every month and that means you will have to stop the external filter every month which I do not like. From my personal experience I do not recommend an external heater, compared with the benefits of an external heater the disadvantages are much more and that is why I do not recommend this product.