Imo 10ppm is optimal but 20ppm is normal. 40ppm is high, but anything >40ppm definitely becomes dangerous especially over time.
Unless you have Discus, this is a good general guideline. My GBRs and Cardinal Tetras do fine at 10-15 ppm.
Some species, such as Goldfish, will do OK over 40 ppm. But it's best to not go there if you can avoid it. As long as your tank isn't highly overstocked, a weekly 20-50% water change should keep you below 40 ppm.
Imo 10ppm is optimal but 20ppm is normal. 40ppm is high, but anything >40ppm definitely becomes dangerous especially over time.
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