Try frozen peas . . . take a few out of the bag and let them soak in some warm water for 2 or 3 minutes and then gently squeeze them so the outer shell (or skin) comes off. Throw the shells away and feed the insides.
Blanched zucchini/cucumber and broccoli will work.
You can also try unsalted green beans and canned corn. Most fish usually prefer the peas however.