Why is my goldfish laying on the bottom of my tank?

Fancy goldfish are highly prone to swimbladder problems due to the shape of their bodies. The problem with the swimbladder isn't a disease itself but can be caused by internal bacteria infections or by taking in too much air when feeding, especially if you feed floating foods. Another cause is bloat, which is due to feeding too much dry food. What I would do is cut out the dry food and feed some frozen and live foods such as bloodworm and daphnia plus some vegetable matter like boiled peas (with the skins removed). If this fails transfer the fish to a seperate container with just enough water to cover the fishes body. Try rasing the temperature slightly and add an anti-internal bacteria treatment.