i also keep discus, i have had no problems, we have very hard water, i do about 30 to 50 percent water changes weekly. sounds like a lot, but it is a sunday routine. take a shower first, you may not have hot water left if you have as many tanks as we do!!!
i keep a school of clown loaches, and the husband through a couple of "yo yo's" small geo's. rams, and i think that is it.
strongly do not recommend rays, it's okay until they get as big as my rays, they make a dinner out of them!!!!