I use garden hose for WC. When I purchase a new hose, I use it to drain my tanks a few times before I use it to fill the tank. I am concerned about the manufacturing residue inside a new hose, and draining a new hose many times its volume assure that all residue and leachable chemicals are gone.
I am convinced that all plastic ornaments and plants leach out plasticisers as I can smell plastic strongly from new pieces. I used to have natural rock caves for my Tanganyikan cichlids. But I got tired of rearranging the rock so I replaced them all with plastic rock and caves . The Tangs started to die off slowly until I took all the plastic out, did a large WC, and put back the rock. None of my Malawian and CAs were affected. Another time I glued two pieces of rock together with GE Silicone II. I was lazy not to soak the glued pieces after curing in water for 24 hour before putting into the tank. By the end of the day, a group of Tang A. Compressisep I raised for over a year started to die off, but none other fish were affected. Apparently, Tangs are super sensitive to chemicals that may be harmless to other fish.