Looks like a Swallowtail caterpillar. Personally I would put them back in my garden.
Also, depending on what species and what they are feeding on, they could be poisonous. I would not feed them to my fish.
Eheim Pimpette #270
Eheim Pro II 2028; Classics Series -2215, 2217, 2260, 2262; Pro 2229 Wet/Dry