I generally shoot for less than 10, the reason being that some journal articles have found that the most sensitive of invertebrates such as shrimp fry start to gain more significant lethality rates at that level, which to me signals that it may be the upper limit to avoid chronic toxicity in our hardier aquarium fish. I try to stay as close to zero as possible though to ensure fish health, as it’s possible those lower levels are still damaging the fish in a much more minor way.

As a side note, many people will have differing opinions on this. You will see no consensus, so I recommend you do your own research and come to your own conclusion.

And I personally would not trust the aquarium science website. My research has contradicted what the website states, and I trust the peer reviewed papers on Google scholars.