Feed them more vegetables rather than meat. Too much protein can shorten the turtles life span and can cause organ harm. Feed them things like Carrots, Bell Peppers, Dandelions, duckweed, water lettuce, etc. Bananas serve as a good treat but don't give it to them very often.
There is a pellet food for them to eat. Put it in the water, and feed about 5 sticks for every inch ounce of turtle (what my Vet said) or let them eat until full and remove extra food from the water. Dispose of extra pellets, do not feed them the wet pellets again. DO NOT FEED GOLDFISH REGULARLY!!!!!!!! Goldfish have almost no muscle compared to fat. They can mess with your turtle's liver and kidneys. Goldfish should be reduced to a SMALL one every couple weeks. Sea food is not good for them either, as it is from the sea, not from freshwater.