Jim,
You may have already done the following things, but wanted to chime n my personal experience with the mystery die off of fish and what I learned from my mis.... mist..... mista.... well....from my past.
I would check the water params (nitrates, ammonia, salinity) first. Things not right here can cause stress. The stress could very well be caused by too many fish competing for tank space. I know it sounds like the old "you got to many fish" speech, but fish really do stress under such conditions. Kinda like when the whole family is over for Holidays, but decide to stay a few extra..................weeks! Things are cool for a while , but then you realize why we all moved away from home.
So the fish get stressed.......show some signs of sickness.... maybe one gets weak and is not able to defend himself, then is killed..... Then you add maracyn or whatever antibiotic.....which if not carefully calculated (based on dosage per gallon (not of how many gallons the tank is , but how many gallons of water you still have in there after displacement from rocks, substrate etc.) If you add too much it almost surely will deteriorate the bacteria colonies (good ones and bad ones) in your tank, which further complicates the matter by created an greater imbalance in the system.
Pain in the butt.... you bet. My suggestion is to get all the family clearer out...clean house.....and hope your immediate family recovers.
So get down to a reasonable amount of fish.... Do a good water change....add some carbon to a bag and toss it in the tank for a day then change it out and repeat for a few days. Stop feeding, if you still are.
Best case scenerio is to take the sick fish out and put them in a quarantine tank so you can watch and medicate them, but few people have the equiptment of willingness to go that route. Fish will normally recover from things like ick and eye issues if given the chance.
Just rememer that too much of a good thing is bad. I love my kids , but if I had 15 of em in here, we might not survive.
