Howdy,
I think it might very well be the fault of fish farms. Back in the old days, demand was low, fish were bred in small, mostly family owned farms. Today, it's a much larger hobby, fish farms extended their breeding greatly. And with increased fish populations, they needed to use more medications. That, in turn, is known to weaken the immune system. I am not surprised about the poor quality.
However, I bet the quality of wild-caught fish improved greatly with better infrastructure (so did availability)
HarleyK