What Lisa said - often there is no one single cause of illness, but multiple factors stressing the fish and making them more susceptible to disease pathogens which are always lurking in the environment. 30 gallons per fish does not seem unreasonable but if they are getting to be very large fish (goldies can get to be 10-12 inches easily) then yeah, your system has to process much more fish waste than when they were small.
Which might usually be OK, but then..... if combined with other factors (high temps - is 85 the temp of the water?) you might find fish more suspceptible to disease than they might otherwise be if they had say 100 gals per fish and more filtration/oxygenation.
If 85 the temp of the water that is indeed pretty high, the oxygen content of water becomes much less the higher the water temps get. You might want to invest in a reflective mesh shade cloth if the pond is getting sun and also get pond aerator going - better yet if the aerator itself can be in air conditioned house or cool basement or garage with the tubing going out to your pond so the cooler air is going into the pond (the Tetra aerator I have has pretty long 20 ft tubing for this very reason ). The lack of oxygen would stress the fish plus allow anaerobic (bad) bacteria to get a foothold.l