Plenty of people use tapwater for discus, especially with the proliferation of tank bred specimens. It can be done without too much concern. We dont keep fish, we keep water, so as long as you keep your tap water pristine I dont foresee any problems. However, the tank MAY or MAY NOT be an issue. Its more to do with dimensions than volume. I have seen people keeping small groups of discuss in a 3 foot tank with no problems at all.