You did not say what type of fish or what they were feed. My guess is you feed them flake food with a high level of plant material, thus black poop. Nothing to worry about, but it is good you noticed it as you're obviously paying attention. Mollys, platys, guppys, gold fish and the altime king of poop and black poop are Plecos.
I have one swordtail and three platys. I do pay attention to them. They always act a little odd in the morning, but as it gets later they calm down. They get hyper again when I feed them (I feed them TetraMin Plus tropical flakes with shrimp). When I first got the green lantern platys, the swordtail kept stealing their food, but now they are all getting enough.They each eat around 4 flakes.
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