3 weeks suggests a disease from the fish farm. Generally, when I get new fish, I quarantine them for 6-8 weeks, or longer. In that period, things like you are describing can happen. It's generally nothing you're doing - it started before you'd even seen them.
It's unlikely to be genetics, and store fish are young. They breed them and move them to market fast. But there is a host of bacterial or viral illnesses that can hit them after the stress of shipping, the store and then being brought home. There isn't much we can do about most of them, except wish we could do something.
P. normani is a generalist with its needs - it can thrive in soft or medium hard water. It doesn't like its water too hot, but that wouldn't be a cause here. I used to keep wild caught normani as I did work with an importer, and they were always mixed in with other species. Once the farms got hold of them and started mass producing them, the fish we get are nowhere near as hardy, and are harder to breed than the wild ones.
I bought half a dozen farmed normani in the autumn, and only 2 are still alive. I have about 20 babies, and I expect they'll be long lived, hardy fish like their wild ancestors. But I bred them quickly because I was worried the store bought ones would fade out. When they're healthy, they are hardy, but getting healthy ones can be a challenge.
