A lot of people use pumpkin, zucchini, cucumber, peas, spinach and a few other fruits/ veges for their fish. You can blanch (partly cook) them before feeding to the fish. Some fish prefer softer foods and others are quite happy to eat the raw foods.
You can buy seaweeds at some Asian supermarkets or in the Asian food section at normal supermarkets. It's available in dry, fresh and frozen.
Make sure any fruits/ veges are free of chemicals.
Wash them well before use and if possible, peel them.
Only add 1 or 2 thin slices to the tank and remove them after an hour.
Do not feed potatoe, onion or onion relatives (garlic, spring onion, shallots, etc) to fish.
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You can also buy vege flakes/ pellets from most pet shops.
