Decades ago, we had a big tank full of the Red-Tailed Tinfoil Barbs. They were so cool. Beautiful. But, our first water change after installing a whole-house water softener (loaded with Iron-Out crystals), I forgot to switch the new softener's bypass valve. Oops! Huge mistake. All our lovely tinfoils went belly up. It was a disaster. I saved a couple of them, mostly by using a hose to force fresh water over the gills. It was the only time I ever gave mouth-to-mouth resuscitation to a fish.