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the green stuff is fine, its probably cheaper and easier to obtain too. i feed my fish nori (the stuff the japanese wrap sushi in, its just sheets of green seaweed), which you can buy at asian marketplaces for a lot cheaper than the seaweed marketed for fish.
i take mine out in the morning and put new stuff in. so it is only in there for 24 hours but i think that might be too long becasue when i wake up it is already falling apart.
you really shouldnt be putting in more than your fish will eat in a couple hours, otherwise, like WarOrks15 said, it falls apart, and your fish wont be grazing it during the dark hours anyway.