Goldfish Not Very Lively

[Yao]

My mom has a few goldfish in a 20 gallon. One of the black ones is floating around, acting very weak. Sometimes it sits on the bottom of the tank on the pebbles, and when it tries to swim a little, it looks like it's struggling. Before that, it was floating near the top all the time and struggling there. All the other ones seem fine. Does anyone know what's wrong with it?

[Zagz]

Is it floating on it's side at all? Does it seem bloated? Goldfish are succeptible to swim bladder disease. You can try some epsom salts and peas for food. Use frozen peas, skin them and put them in the tank. If the goldies are small, mush them up to bite size morsels. Another thing to consider is goldfish are messy fish and may very well need extra water changes. Can you post her water parameters? I'd also do a pwc right away as that can make a huge difference. How big are the goldies?