I had one albino cherry barb, one bamboo shrimp, one guppie all die the other day. One of the cory got stuck in the powerhead and was laying there being eroded by disease that I thought he definitely wouldn't make it. He pulled through and is looking like he will become a champ.
I suspect there is a fungal or bacterial infection in the tank. Dosed it with meds and waiting to see what happens. The reason I suspect bacteria is because most of the small to medium sized fish have shown a lot of signs of outer infections as well as stringy clear poop. I've been feeding them medicine as well for internal issues.