Oto died, now Honeycomb darting about
 

Today one of my otos died. It's eyes were very sunken in, as well as it's stomach. It laid at the bottom for a few hours before dying. Now, just now, I saw one of my honeycomb cats dart around furiously, swimming into all sorts of things. He looks a little pale and he's breathing heavily. It looked a little like he was trying to scratch himself, but this literally just started, so I can't tell. All the other fish are fine, except for the one that died of course. It may just be that he got spooked, but should I be worried? What did my oto die of, and is it contagious? Luckily water change day is tomorrow.