My fishkeeping life is so much easier with low hardness, low pH tap water.
For a frame of reference, right now in one of my QT tanks, I have some tetras and Corys that all came from the same set up at the dealer's place. So I QTed them together. There's something, I'm going to guess a virus, working through the tetras. The Corys seem fine, which suggest it's viral. A bacterial problem would kill everything.
The dish have died, one or two every three days or so over a month now, with absolutely no warning symptoms. I've sat and studied the fish in there looking for clues, and they all look great until they don't. So I keep a hard quarantine on the tank and remove the deads as they appear. I am absolutely powerless.
The water is good, there are still 20 tetras and 6 Corys in a 75 gallon planted, the food is good, the heater is reliable... The tank beside it, with no farm raised fish but identical size, decor, filtration, water and temp is rolling along just fine.
I've only lost one fish this past week, so I hope the outbreak is burning itself out.
I could throw all kinds of meds in there, but it wouldn't matter. I've worked in the fish business and I've seen this pattern before. The fish are suffering from ^%^$ #$#@%&^ ^&^%, or possibly ^%^$ #$#@. With luck, I'll finish up with a smaller group of fish. With poor luck, I will have a Cory tank with no tetras.
What we don't know about fish diseases is way more than we do.
