What can kill one healthy fish every 2 weeks??

[RJT]

I have a 220g with 20+ fish all eating well. For some reason the past few months I lose one fish every 2 weeks that was eating well the day before and acting normally.

There are no signs on the fish and no heavy breathing. Just slow breathing and they just lay there. I take them out put them in a paraguard dip and they die within a few minutes, or I put them in a bucket with tank water and they die or I freshwater dip them for 12minutes put them in tank water then they die or I make new water but they still die.

What kills so often but leaves no marks?

[Newsmyrna80]

Have you added anything new to the tank? First Paraguard will cure nothing. Second, twelve minute dip is too long.