Back when I used to volunteer at the Vancouver Aquarium and fed the fishes, we often used herring. I think I can clean it and chop it up into 1inch steaks in about 3 seconds.

edit: when I re-read this, it sounded weird that I was feeding fishes with fish, but they eat them. Along with smelt, shrimp, and occasionally clams. The resident octopus got a whole dungeoness crab - alive...