Actually it's a great thing to add to their diet. Too many people feed only flake & frozen food, which can lead to digestive problems in many fish. Veggies solve this problem, pumpkin is in season now, time for me to pick one up.
I've fed my fish peas, carrots, zuccini, cucumber, green beans, nearly every vegetable I've run across. I have an oscar that loves zuccini, and really pouts if he doesn't get a spirulina tab every evening.