Discus tetra, Brachychalcinus orbicularis
Almost all tropical fish will need some means of maintaining a constant temperature within the correct range for them to thrive.
There are a number of thermostatically controlled heaters available to suit this task. Heaters are made in different sizes and selecting the correct size is very important. To small and it may struggle to maintain the temperature, to big will give you no time to react should it "stick on" before the fish are boiled. The table below is for guidance only, rooms which are very cold may require the next heater size up.
Heater size (watts) - Aquarium size (Imp gall - Litres - US gall)
75W - 15 UK gall - 68 Litres - 19 US gall
100W - 20 UK gall - 91 Litres - 25 US gall
150W - 30 UK gall - 136 Litres - 38 US gall
200W - 50 UK gall" - 225 Litres - 63 US gall
300W - 80 UK gall - 364 Litres - 100 US gall
2 x 200 watts - 100 gall UK - 445 Litres - 188 gall US
2 x 300 watts - 150 gall UK - 682 Litres - 188 gall US
Most modern heaters and thermostats are manufactured to a high standard and are very reliable. They mostly rely on a bimetal strip, which can be manually adjusted to the particular temperature required. There are now electronic versions available which are accurate to within 0.25 degrees C.
Some combined heater/stats come with a temperature scale, this should never be relied upon and the temp should always be measured with a good quality reliable thermometer.
99% of the time a simple straightforward combined heater and thermostat will be all that is required. These are available in every fish store up to about 300 watts in size.
When two heaters are required for very large aquaria it is better to select separate heaters and a single thermostat because trying to adjust two separate thermostats to come on and go off together in a single tank would be almost impossibility.
Some large power filters are now being made with a built in heater and thermostat. This is a real step forward because there is no need to try and conceal one in the tank itself and no fish will accidentally get a heater burn. There is also a disadvantage that is if the filter breaks the heating will also be lost. So make sure you have a spare impeller and any other parts that might fail.
Then there is under-gravel heater cables, which are generally quite low, powered and their main use is to create convection currents in elaborately planted aquaria and to encourage good root growth.
Finally there are under tank heater pads again these don't need to be concealed inside the tank, but they can only be used where an under-gravel filter is being used or there is a risk that the tank base could crack due to a hot spot forming. Unfortunately my experience with these pads has been very poor; I have used three in all and every tank ended up with a cracked base.
When you think of the value of the stock in a large aquarium particularly when fish like Discus are concerned it is false economy not to take some form of precaution where the temperature of the water is concerned heaters can fail by sticking on or by the element failing. To combat this there are electronic temp alarms, which will give an audible signal, when the temp goes out of a pre programmed range. These cost about 1/3rd as much as a single adult Discus! Even cheaper are large digital thermometers, which have a probe in the tank, and a large digital display, which could be positioned, so that it is easily seen when given a quick glance and so act as a visual warning. Better still use a double thermostat this will act as a backup in case the first one fails. See diagram below for details
Mercury or alcohol thermometers are extremely reliable and accurate and these are the preferred option. The stick on digital thermometers are extremely inaccurate and will vary more with the room temp than the tank temp.I have yet to see the scale on the thermostats be anywhere near accurate. Temperature is very important make sure you don't get it wrong for the sake of a very reasonably priced thermometer.
The suggested wattages are a guide only, obviously if a tank is left in a cold unheated room then a little more may be required. But if the tank was insulted along the back pane and the sides it would make a significant difference ito the required power. Or on the other hand if the tank is kept in a permanently warm room then less power would be required.