It is pretty high. However I would like to note that, at one point, went up to maybe 50 ppm with no noticeable adverse changes in behavior or activity to any lifeform in my tank. my hermits, snails, mantis, featherdusters, and damsels were all fine and there was no "heavy breathing" from my damsels. Of course I did an emergency 30% water change right then and there. I have a 10 gallon by the way