Just a shot in the dark, but were the mussels you fed them fresh or previously frozen? I wonder if there was something in the mussels that isn't agreeing with them? I usually freeze mine for a few days then thaw in tank water...just in case there are ickies in them that might harm the puffers.
I'm sorry you are having such troubles, Roxy
. It sounds like they were a bad batch, it happens sometimes.