Apparently Sick Goldfish

Question
I have an apparently sick common goldfish. It has eaten almost nothing in about a month and spends most of its time at the bottom of a twenty gallon tank. It's the only fish in the tank and it's about eight inches in length. It was eating flake food for about the last ten years until it suddenly stopped. I tried feeding the fish some peas, thinking it might be constipated but it won't eat them. The tank has a moderate algae growth. About 25% of the water is changed about weekly and the filter is changed just prior that. I'm not certain about the water quality but I know the tank needs to be thoroughly cleaned. However, that's my son's responsibility because it's his fish, although he's been to the house maybe two times since the fish's behavior changed but hasn't done it. At this point, I'm writing to you because I don't know what else to do for the fish.

Answer
Some mistakes are that the tank is way too small, the filter should never be changed, and you should increase water changes. Goldfish need a lot of vegetable matter in their diet so try getting it something with veggies and reduce the flake food. Remember Goldfish need to be fed 4 days out of the week.

- Ash