what i have seen for freshwater vs. saltwater, ...
freshwater lives a precarious life, always changing, water levesl change drastically,
many species have addapted to a life of existance, making it through each year to approach winter and to adapt again as streams run dry, or freeze over, ... such is life when everything changes.
saltwater things are consistant year after year, month after month, evolution does not need adaptability, it needs survivability, to be smarter than your prey, and to be smarter than your hunter. the water does not change enough to spend evolution worrying about changes in the environment.
for freshwater to have tanks of drastically different characteristics shows a versatility of evolution for a species that has survived only because it is adaptable to a very large variety of changes. ... indeed some species going as far as to use those changing seasons to it's advantage for propagation & reproduction.
so a tank does not need to have exceedingly strict parameters, ... for the most part
freshwater evolution has timed itself to be attentive to the changing water conditions, as water conditions change and reach certain optimal ranges breeding seasons are started.
temperature changes (season), cleaner water (large rain falls), ... falling of leaves into the rivers, ...
the optimal for breeding does become strict though, ... and without the tank ever reaching those ranges how safe & happy are the inhabitants ?
many species can exist in zoos on basic needs, ... but breeding requires more then the basics