They can get away with some fish, but not all fish, because some fish are more tolerant than others. It also helps if one does frequent WC to dilute the toxins to harmless levels.
I used to place some plastic ornaments from Petsmart in my tank, and my Tangs kept dying until I removed the plastic. We all use plastic in our tank, such as our filter intake and parts. But not all plastic are the same, some leach out more chemicals than others. A few years ago, there was a recall of certain baby bottle made of plastic that was unsafe. Those that smells strongly should be avoided even if it is made for aquarium use. The toxic effect may not be immediate, but can harm or kill the fish in chronic exposure.
