just because they have been captive fish for even 50 or 60 years doesn't change tens of thousands of years of living on a specific diet. vegetarian fish fed what you have listed will have shortened lifespans and be more susceptible to health issues. plus if they require a vegi type diet, they'll be healthier and happier with food that has a much higher fiber content than what you're giving them. agreed, you can't always mimic a wild type diet, but you can try.
if you aren't willing to chop up some F&V for them, omega one makes some good vegi foods.
like duanes said, try some leafy romaine. stick a whole leaf in and watch them tear it up. finely chopped zucchini would be good for them too.