Whats wrong with my cory cats?

pappy Member Posts: 15
edited October 2018 in Beginner's Forum
About 5 weeks ago I purchased 4 salt and pepper Cory cats all were good I was just feeding them Hikari micro pellets until I got some Hikari sinking wafers first popped them in the tank and bang Cory’s were straight onto it eating swimming all fine. I then had a few s days off for a holiday come back and I have a brownish growth on my driftwoood figured it’s all okay since it’s similar to when I first put the driftwood in but brown, I fed the Cory’s some sinking wafers and they didn’t eat them so after 20 minutes I took them out they were going good for a bout a week when I noticed one swam under the filter outflow and pushed to the bottom of the tank really quick and was then swimming funny it was upside like it couldn’t keep is weight in the water so pushed it a bit and it came good so I left it and went to bed wake up and the bigger Cory was swimming the same belly up I poked it a few times it came good then I want to school come back and it’s dead. Now this morning wake up there’s another swimming belly up on the bottom then it died. What’s going on please help?tbtw the other two Cory’s are fine. Thanks in advance I also noticed one hitting the surface regurely but it was the only which did it my current Cory’s aren’t eating either and there a bit skinny