Feed him plain (no salt), freshly cooked peas. No canned peas. Bags of the frozen variety that you can microwave are the easiest. Once cooked & cooled, remove the entire outer shell & mash the pea between your fingers into bits. Offer 2-3 peas to your goldfish 2x a day for the next 3 days- no other food. The shell poses a choking hazard & goldfish have a difficult time digesting it, so be sure to remove it. Then slowly introduce a quality sinking pellet. You should also be offering daily cooked veggies as part of his diet well (broccoli, spinach, zucchini, etc).