many sites will quote lifespan as an average in aquaria. as we know, some die quickly, some dont.
as an example, arowanas can live to 25 yeras plus and there are examples of fish in farms that life around this age, and still going strong. but in asia, in the hobby, arowanas die much sooner, i think due to severe under filtration and constant exposure to toxins.... they may be ok, but its like a human, if you area smoker/drinker/drug taker, your likely to live a shorter life. there are always exceptions to the rule.
to fish i think toxins are just that, life shortening.
with discus, i see many breeders that refuse to accept that a good automated water change system is good.
only when they see signs of exposure then they change the water...
so imagine, if this was a human, dog, cat? you wait untill the animal is uncomfortable before you do something about it, instead of prevention? it doesnt make sense to me.