In the 7 years since I wrote my book I have attended a few conferences put on by neurologists. My wife has MS so we get invited to these things all the time. The last one was about stress and how it lowers our immunity and gives us all sorts of other afflictions including what I consider the most interesting, shortens our (and I assume fish) lifespans.
How does stress do this? It is actually simple. Our bodies make Cortisol. We exude it from our adrenal gland constantly and it gives us energy for various tasks. Our bodies use it as the day goes by.
If we for instance are being chased by a rhinoceros, Model T Ford or Henry Winkler, our Adrenalin gland would pump out a lot of cortisol to give us energy.
Cortisol dissolves some of the calcium in our bones for our energy. and turns some muscle into sugar for energy.
These are good things especially when we were always in danger of being eaten by something.
But cortisol has some big problems to. When our bodies produce it but can't use it it also brings out dormant diseases, lowers our immunity, lowers our sex drive and a whole lot of other things. But the worst thing it does is shorten our life span.
The way it does that is because there is this thing on the ends of every chromosome in every cell of our bodies. This thing is called a telemere. As we grow and age our chromosomes need to divide and different creatures do this at different time spans.
The telemere at the end of each chromosome is like the "aglet" on the end of our shoelace and it keeps that chromosome from unraveling so it protects it. But every time the chromosome divides the telemere gets shorter until there is no more of it left and the chromosome degrades and can no longer divide. That is the end of our lifespan and in Humans happens at about age 80 or so.
That is why everything has a lifespan and can't live forever.
Cortisol is called the stress hormone because stress causes us to make more cortisol and if we are not physically running for our life, our muscles can't use that cortisol so it builds up.
The main problem with this is that it shortens those telemeres shortening our and I assume a fishes lifespan.
Fish experience stress in a tank but really experience it in a tank not set up like the ocean such as a bare quarantine tank with PVC elbows from Home Depot and fish not getting the food they recognize.
So (and I am speculating here as I am an electrician and not a fish physiologist) if a fish has a normal lifespan of 10 years and we get that fish 2 years old. 72 days in quarantine with copper will make that fish create a lot of cortisol which will shorten it's lifespan. How much? No one knows because all fish are different and fish are built much differently than most of us.
But I think that is the main reason that I have not been able to find even one fully quarantined tank or fish that is very old. But of course I am guessing.
If all of our fish are not mainly dying of only old age, we are failing and doing something wrong.
Just my opinion of course.