In my opinion you don't need a separate water tank to prepare water in. That is a lot of hassle.
Depends a lot on where your tap water comes from. Where do you live? Most people in NZ will have quite soft water but you should find out.
If you have soft tap water: (most of Auckland)
Do water change straight from the tap with a mixer (find the right temperature, this is important) and add dechlorinator to the tank as you refill.
At normal stock level and if you get discus bigger than 8cm do 30-40% water changes pr week. Never more than 50% at one time unless you know the water you are putting in has same parameters (Temp, pH, TDS) as the tank water otherwise the Discus might just drop dead from shock/stress.
I keep wild caught Discus in west auckland tap water (very soft and relatively low TDS) with no problems.