If a fish is willing to breed I take that as a sign that they feel as comfortable as they do when breeding in nature.
Hello; I have been thinking about mating being an indicator of acceptable water and/or tank conditions for some time. There was a time in the dim past when I felt this to be an absolute. Back when I was attempting to induce various species to spawn, I was often having difficulties providing a successful breeding tank setup. There were some species that no one had been able to induce to spawn in captivity. Other species were difficult, but some had managed it. My interpretation at that time was that tank conditions in which fish would spawn must be considered "good" or even "pristine". Now days I tend to use terms such as "adequate" or "minimal". A difference with significance to me at least.
A couple of observations that might possibly support my down grade from good to minimal. First, back in the 1960's and 1970's the Cumberland River that has its head in Harlan County would often run black from coal mining activity. There was also a lot of sulphur released as well. There also was then and likely even now, a lot of straight pipe sewer lines running into the rivers and streams. (Ask me how I know about this) The water quality was by no means good, but yet the river was not dead.
For some years I thought that there must be a few tributaries with good water that acted as a safe haven the fish could escape into. Now I am not so sure this is the only answer. I have talked to people who pulled fish from the river during these times.
I now consider it possible that the natural imperative to continue the species overrides the need for ideal water conditions. I suspect that fish will spawn under less than "good " conditions. It may well be that the eggs or fry cannot endure the conditions, but the adults do seem to go ahead and spawn.
Let me speculate further. It may be that water quality means less in those species which do not spawn in captivity than some other trigger. A guess, perhaps there is some sort of insect hatch that has to happen to trigger a spawn??
One last analogy and a poor one. I see documentaries about human refugee camps or famine areas from time to time. More specifically, those that have been going on for many years. In all the filmed and reported cases there is always a portion about the suffering children. Condition in those camps and famine areas must be horrible, yet it does not prevent mating??
At any rate, I am no longer so convinced that spawning is an indicator of "good " conditions.