Losing Guaramis!!!

[small fry]

OK, so i was just walking out of the door, i ran downstairs to feed my fish, because i was going to be out for a while, i feed them, and notice that one of my dwarf guaramis has a bloated area right beneath his dorsal, i came back 4 hours later, he was on the bed and it seemed as though that spot had exploded, and all of his scales were protruding out like a pinecone type scenario, i lost a second to the exact same thing today. I was unaware of anything being wrong with him, as he mostly darts into the house(refer to avatar) when i come by, (of course that could have been some kind of illness all on its own) My two remaining guaramis show no signs of timidness, and go straight to the glass and follow me around when i walk by or when it is feeding time. They also have nice bright colours, no deformities, and they seem bright and playful.
So what was (or is) in my tank that is picking off guaramis and none of my other fish?? thnx for readin

-Josh