Most pet shops sell a frozen food called Marine Green. It has prawn, fish, squid and algae or spinach, depending on brand. You can also add plant matter to their diet. Marine algae or Spinach are often used. Make sure the spinach is free of chemicals and wash it well before adding to the tank. you can dip it in boiling water for a few seconds to soften it up. You can cut it into little bits to make it easier for them to eat.
You can buy vegetable flakes or pellets from pet shop and feed these in addition to other foods.
You can grow Duckweed (floating plant) on the surface of the tank and the fish can eat that when they feel like plants for dinner.