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Spinach is very good as it contains iron and vitamin C (essential to fish health). Water cress is also good in this respect. Blanch them briefly in boiling water to soften the outer cell wall, which makes them easier to eat, but don't cook them through as you lose some of the nutritional value.... no-one cooks a wild fish's veggie matter
Some fruits may be taken, but aren't a natural food for many fish. Wild fish in the UK enjoy Elder berries, so if available, these may be a possiblity