for starters, what are the basic specs of your tank? size, age, water conditions, etc. That info will help people better understand your setup and make it easier to help.

I have a 30 gallon long. It has been set up for three years. Prior to that the same fish were in a twenty nine gallon tall for about 4 yrs. I have (had) 5 Black skirted Tetras, one left, 2 Gouramis, 1 Corey cat, 5 Danios, 2 Some other Tetra that I can't remember the name at the moment. On Friday I noticed the dead tetra, scooped it out. Figured old age, Saturday, dead Tetra, scooped it out, began to wonder, checked water, it was fine so were the other fish. Sunday, two more acting funny. Bulging eyes, film over eyes, seemed to send thenselves into the gravel. I went ahead and did a water change. By Sunday evening both of those Black skirted tetras were dead. The last remaing tetra seems fine and looks normal. As I said, I have never had anything like this happen before. I didn't bring in any new fish, didn't have an ammonia spike, filter didn't crash. The only thing different is a new kind of algea wafer that I got for the cat fish that the gouramis go after rather than the cat. I don't think that has anything to do with it.

Possible the tetras ate the wafer and it expanded? Maybe just a bacterial infection and this will be the end.

Didn't see any of them going after the wafer. They usually just float till the food drops in front of them. My DD thinks it may just be old age. Possible since I did get all of them at the same time. I believe the last remaining Tetra, which is smaller, was given to me about two yrs ago. Could explain why it seems fine.

It sounds like Pop eye to me it is caused by a bacterial infection.

Have a look here

http://www.aquariumpharm.com/disease.html

Keith

Yes, guess you are right. By the time I saw mine, not sure meds would have helped.