What's wrong with my goldfish?

My fan-tailed goldfish began getting black spots and markings at the tips of his fins and tails and his belly began blotting shortly after he had finrot. He will swim around when there's movement and he eats but when not he hids behind a rock and hangs out near the bottom. We recently changed our tank's water to reduce the amount of certain chemicals, but he doesn't seem to be thay much better.