You're welcome.

Could be black spot disease. Although I've never heard of a fish sinking with black spot. The treatment for black spot is easy. Get a good anti-parasitic medicine at your pet store. Follow the directions on the bottle for a 5 gallon tank. If you have a hospital tank, now would be a good time to move him to it.

If you feel it's time to let him go, please don't flush him. It's a very inhumane way to do it. Place him in a ziploc bag full of his fishie water and a few ice cubes and place him in the freezer. He's cold-blooded, and the cold will just kind of put him to sleep slowly.

Now, about the sinking problem...can he eat?