We often see a quibble about how much nitrate is 'acceptable' in the fish tank. Although not as lethally dangerous as ammonia and nitrite, nitrate is a poison to fish. As Byron documents, there is so little nitrate in 'wild' fresh water it can not be measured.
There would seem to be little point in attempting to debate how much poison [nitrate] is lethal in time.
Anything we can do to keep nitrate levels as close to zero as possible is to the benefit of the fish. For some, that means little more than routine weekly water changes of enough volume to dilute the pollution a sufficient amount. Others may require [more] plants, rigorous tank/filter maintenance, reduced feeding, reduced stock levels, source water correction, deep sand, de-nitrate filtration and/or other means to keep nitrates at bay.
The worthy objective of every fishkeeper is to achieve and maintain the purest water possible in the tank.
With very high nitrates in my well water, I fight the fight every day.