WHAT temperature for discus fish?

Your heater could be the cheapest piece of equipment you purchase, but it could potentially kill your fish the quickest. Discus need a constant temperature, constant is key but it should be between 28 and 30 degrees C (82 to 86 F).  Outside of that range for any amount of time can cause issues.  Above 35 and your fish can quickly die.  I've seen a tank go down to 18 due to a heater failure and they were ok, but wont be for long.

Most heaters come as one unit and go directly in the tank and plug into the mains.  The temperature control and thermostat are built into the heater. For nearly everyone of these heaters I have used,  the on heater setting system is very inaccurate.  They can keep the temperature stable but you will need another accurate way to monitor temperature.  Laboratory thermometers are very good for this and usually accurate to within 0.5 to 1 degrees. 

Other more advanced heaters have an external controller and a temperature probe.  This can be placed in the tank away from the heater providing a more accurate control of the tank temperature.  These probes and controllers I find more accurate and when switching to one of these found I had been keeping the fish a few degrees out for some time.  The external controller is used to set the temperature and will even alert you when the tank is outside of your set parameters for temperature.  Highly recommended, but will cost you more. 

TOP TIP - A single heater will either set to heat or no heat when it fails, so either way the outcome isnt good.  I have had a large heater in a small tank stick on and the fish died very quickly.  You can limit the possibilities of this by running two heaters, half the recommended size for the tank.  If one of these should fail you will have longer to notice and fix the issue.  Its very unlucky for both to fail at the same time.  

Many websites will recommend a heater size per volume of water.  Remember you must maintain the water temperature accurately all the time.  Don't buy cheap heaters and always have a spare available.