We have a 60 litre tank with around 6 adult fish, and 6-8 juveniles. They are mostly happy, rarely swim at the surface, and spend much of their time foraging. On the rare (maybe monthly) occasions I put a block of frozen bloodworms into the tank, they eat it very quickly and appear to enjoy the change from bug bites or algae wafers (for the 2 bristlenose catfish). However, after around 30 minutes the fish appear visibly bloated and will swim at the surface for some time. Within a few hours, they will revert to their normal shape.
What is it about bloodworm which causes them to become visibly enlarged?