A pH of 7.8 is usually an indicator that you have sufficient calcium in your water for Crays, although the best way to check, is to measure alkalinity, (high is good 80-100, low 40 or below my be lacking) this also means not letting pH and alkalinity drop much in between regular water changes.
Adding some baking soda with a water change will help boost alkalinity.
If pH and alkalinity drop more than a couple points between water changes, it means calcium is being used up buffering the tanks metabolism by products, and more water changes may be in order to maintain that calcium.
I keep an endemic vampire crayfish with my Andinoacara, and though it is a filter feeder, it will occasionally try to grab a cichlid, but they easily are able to shake off, don't know that a discus would have that ability before becoming injured.
I tried keeping more than 1 cray, but they fought, and when one was injured, the cichlids would swoop in as a pack, and finish it off, tearing it to bits.
These Crays were at least as big, if not larger than the fish themselves, one is bottom toward left hand side below.
They are called vampires because of the two white filter appendages that look like teeth, but aren't.