Hi John .
About the water changing . How much % do you change and how often?
Maybe you can try to fit a 30 gallon tank for ageing In your tank cabinet and that way you'll be able to perform big WC with half aged and half straight water with less or no harm at all to your discus .You don't have to age the whole % of the WC .
If you can't age any water , Period , than I guess small and more frequent changes would be the best bet for you .
About the noneating discus , maybe you should run a course of 10 days treatment with your seachem metroplex medialcation along with the higher temps . 32 C and see if that helps .
Be patient though ,cause non eating discus could take awhile until it regains its appetite back . it could take up to 2-3 weeks after you start medicating until you see its first bites .
Good luck John and keep us updated with your progress .