From what I have read, it seems the "weaker" fish are partly/mostly a result of inbreeding. As for testiing, I never test any more but I did when I was getting my tanks up and running. I think the 2 go hand in hand though. Because of the weaker fish (what ever the reason may be), they aren't as tolerant of bad water conditions. That's not to say that people back then had bad water parameters on a regular basis but more that the fish were able to tolerate the ammonia and nitrite spikes in the beginning better than they can now. Even then though, as I mentioned in another thread, I'm sure a lot of people tried the hobby back then and were turned off because they kept losing fish and didn't have the resources we have today to figure out what was causing the deaths and how to fix the problem.









