I've found at least for ich in new shipments of fish high temps are good. I usually aim for 85-86, not 90, but a week at that and then dropping it down seems a good therapy. I've tried to leave a tank like that for a while and they started going downhill quick. Seemed like the ich went away in the first week, they looked good the second and quit eating and lost weight the third or beyond. Bringing them back to normal temps over a few days seemed to fix it and they were without ich and started eating again.