They should be able to spawn OK, but the hatch rate may be low. I've never bred discus in hard water. I'm only going by what I have read. My water is from a well, very soft, with a PH of 6.0 to 6.2, so I've never had that problem. I say go for it and see what happens. If the hatch is a bust, you could always add some RO water for the next spawn. After the eggs hatch and are free swimming, then you should be in the clear. After all, once discus start to spawn, they will do it regularly for a while. Pairs will often lay eggs 10 or more times before they stop.