So 10-15 is a safe level for the fish?
I dose 45ppm a week as KNO3 and feed like mad and have a high stocking level.
Fish have breed a dozen times, the tank is clean and very healthy.
less is NOT better obviously when it comes to ferts, there's is no factual basis for this. It's plainly put, steer manure. A myth.
NO3 from excessive fish waste and lack of care for the aquarium, this is more of NH4 issue, not NO3.
Since KNO3 never starts as NH4.......they are NOT the same.
Unlike PMDD(which is really what PPS in fact is), EI makes no assumption that less is better.
And unlike many who claim this, I actually test the upper bounds, and folks do make mistakes often times, and report no ill effects at ranges in the 100-200ppm NO3 ranges.
How can anyone who does not test their own claims.......... be trusted and able to support their contentions I might ask????? I have mad breeding, I have breeding of fire red shrimp, fat healthy cards, sell lots of nice plants......tank is clean.
Not just a dozen little fish..........
So in ANSWER to your question, we do not really know the upper bounds.......they are however well about typical dosing errors hobbyists might ever do.
Even if I messed up 5x, I'm still along way off from doing any harm.
A better question is how better to manage CO2, since overdosing CO2, not providing enough current etc, causes 99% of the fish related deaths due to additions in a planted tank, ferts are almost unheard of entirely as far as deaths or stress to livestock.