Most fishes are carnivorous and live on insects, insect larvae, worms, grubs, shrimps, small fish and similar animals. However, almost all fish also graze on live plants, algae and hence considered as herbivores. As a result in the natural environment most fish are really omnivorous and eats that contain both animal and vegetable matters.

In aquarium without any plants and algae, fish lacks vegetable. If you feed you fish with only carnivorous food, they can develop nutritional deficiency and cause many problem like constipation, dropsy etc.

Vegetable contains lot of fibres and helps in digesting food. Regular feed of vegetable can keep your fishes healthy.

You can try feeding many vegetable including green peas, beans, Zucchini, Broccoli, Romaine lettuce etc.

You can feed some vegetable directly or after washing in hot water (Example Romaine lettuce). Fishes like koi, Goldfish will love these foods.

To feed vegetable (like beans, Zucchini) cut them in to smaller pieces and then boil them in microwave till it get soft. Strain the water and cool it off. This is very good for livebearer.

To feed green peas, just boil till they get soft and feed them. This is particularly good for goldfishes.
To feed broccoli, remove the dark green portion (otherwise this can create a mess in your tank), cut and then boil the broccoli till it cools off. This is a favourite of Plecos.

Good Luck

Deepak
[We Love Our Fish Tank]