peas!!!
seriously, goldies love them
Microwave a pea and then let it cool. Remove the outer skin then feed. If the goldies are still little then mash the pea up and give a little at a time. A lot of goldfish owners feed peas once a week as routine. If your fish are showing signs of constipation at present then fast them for three days then feed them only peas for three days.
Also if you use dried foofds such as pellets, flakes, then pre-soak them in some water for 5 minutes or so before feeding. If you feed them dry they can contain a lot of air, which is also bad for thier swim bladder.
Fish will eat other veggies too such as romaine lettuce (blanched) or cooked brocolli. I find peas the easiest though. Preferable to use frozen instead of canned as canned often contain salt.