Not really matter for vegetarian fish. They eat a lot and they do need a lot to eat. Because of the nutrition value inside veggies is much lower than meat, these fish constantly eating to have enough energy.
Best way is to feed them many times a day with a small amount at a time (Not too few). Im not sure how much is the best but after a few try, you should see how much you fish will need.
You should be more concerned with the filtration. As they eat alot, they also produce a huge amount of waste.