My comet goldfish

Question
I got a comet goldfish from the fair a year ago and he has gone through a lot of ups and downs...but he is recently not as active. For instance, when I usually feed him or even walk in the room he immediately jumps to the top and starts  gulping for food. But now he stays at the bottom, and just waits for the food pellets to fall to the bottom..he still swims..but at the bottom. I also notice some white looking bumps on his tail fin. He also has some green-ish grey stuff around his eyes but he camps this way. OH. one more thing wen i first got him for a few weeks he turned black...I prayed he'd get better sand changed his water a lot and he got better...what was it?:( Please help I really want him to live.
DETAILS: His tanks has a top fin filter and its lighted and is 10 gallons. I clean it ever four weeks (change 25%) and scoop out poop whenever i see it.

Answer
Katie,
The minimum size tank for your one goldfish is 20 gallons. Anything smaller does not work. His skeletal structure will be stunted, but his internal organs will continue to grow causing a slow painful death. With a tank that small, the filter cannot filter out the waste that your goldfish is producing. With him in the 10 gallon tank, 25% water change have to be done twice a week. If he was in the right size home, the water changes would only have to be done weekly. Waiting four weeks and only changing 25% of the water is doing little to help him. The white spots could be Ich. You should be vacuuming the gravel every week as well. The color change could have been from stress during the whole getting him and setting up a new home.