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Some time ago, I filmed discus fish fry feeding on their parents. Young discus are ruthless in biting the parent pair.
If this amount of fry is left on the parents for a long time, damage to the skin and fins of adult discus can occur. Young discus fish can simply bite holes in the body and fins of their parents, which happened to me a few times. No matter how much artemia or other food they eat beforehand – then they go on to nibble at their parents anyway.