From The Aquarium Wiki
Other foods suitable for fish include vegetables such as cucumber and zucchini, as well as grazing algae or eating live plants for vegetarians. If you are feeding your fish vegetables, make sure they were not sprayed with chemicals so rinse them well in tap water.
Most fish are opportunists and will eat meat such as blood worm, raw trout, raw Tuna, cooked chicken. These foods should not be a replacement for regular commercial fish food and never fed in large quantities. These foods, though, are very good treats and add more variety to your fish's diet which keeps them healthy.