I feed my fish frozen bloodworms as a treat, and I always thaw with some tank water in a bowl and cut them up into tiny pieces before feeding them. Rainbows, cories, shrimp, CPDs, my betta (he gets tweezer fed, that little baby. All in different tanks) they all love them. Could be that the blood worms are causing an obstruction somewhere and killing them? My rainbows are probably just big enough to eat whole bloodworms, but I do it mainly for the other fish.
Love your tanks and they will love you back