They do need nice clean water yes but a water change isnt need every day. This is only usually the case when you are breeding or your tank is heavily stocked.
When you do your water change the water needs to be as close to the water parameters already in the tank.
Let us know your water parameters etc. Do you know what the water parameters were at the place you bought them??
Discus do take a little while to settle in, they may not eat for a good few days, maybe even weeks (hopefully not). temp them with a small amount twice a day until they pick up. Dont overfeed!