a 2 x 2 x 2 foot tank is around 200 and something litres so you will need at least a 200W heater.
What heater do you have now??
It is also advisable to have two smaller heaters instead of one bigger one.
When doing water changes especially for discus (and with all other fish), it is best to buy a 75L plastic rubbish bin type thing from the warehouse or a 200L food grade barrel (or similiar) and have a powerhead (to keep the water moving) and a heater (set to the same temp as the tank). You leave the water in it overnight and it gets rid of all the nastys.THEN, when you want to change the water you syphon out the old water, and then pump the water from the storage barrel into the tank.
This makes the water going in and out the same temps
It also helps to keep the parameters the same, for example, if you are using something in your tank to keep your KH up you can put the same into the barrel and it will end up the same
This is what i think is the easiest way to keep water parameters and temps constant.
Good luck!!
Rogan
P.S. If you do get discus, i would suggest putting them into a bare bottom tank and keeping the water changes up until they are grown ups (if you buy them small) and if you buy them larger than i would still do it as they dont like to go straight into an unknown environment with plants, driftwood etc.
HTH