I don't think that health is the issue, it's more about whether a person cares to have their fish survive or actually thrive in their environment. We all have to play a balancing act with this, obviously, because of finances and space. Most people try to recreate conditions similar to what the creature originally came from. It would seem that this is the best method to at least know that the fish is biologically thriving, I mean you don't put a polar bear in the carribean and expect him to thrive, but he would probably survive fine if he adapted.I agree about the water differences but would you not agree that they seem healthy?
I'm afraid that doesn't mean a whole lot to me. Just because they do it doesn't make it the right way to care for the fishes welfare. Keeping Goldies & Koi alongside tropicals is also a long way from keeping Goldies with large & temperamental cichlids.