Cichlid Is Sick! Please Help!?

I haven't fed my fish any minnows in a long time until the other day. Since cichlids are aggressive fish I will occasionally put a few in so they can chase them around, keep them active etc. My jack dempsey is about 2 years old. The Other 4 are about 6 months old. I also have a pleco in the tank. All my other fish are doing just fine but my jack, the other day after feeding him minnows (the only thing that has been changed in the tank at the time) started swimming off balance first with his head more downward then kind of on his side. My bf said "oh its normal for cichlids to swim funny when they are playing." I still felt odd about this. His color darkened to almost black. We did about a 30% water change. I woke up to find that he's laying on his side. I think he's dying. I called petco and they said they don't believe anything could have been passed on from the minnows but don't know what else could have caused this. If anyone has any ideas on what could be wrong, please help me. I already have salt in the tank. I add that anytime I do a water change and I've tested my water everything seems to be fine except my jack dempsey!