Is it inside your tank or OUTSIDE?
If it sits INSIDE, it has to be NON-toxic & has to say it on the packaging.
If it's PVC for your filtration system & it's located outside, then it doesn't matter.
You could also use teflon tape instead.
This doesn't make sense. If it's contact with the water, it wouldn't make any difference if the toxic chemical is in contact with the water while it's in the tank or about to be in the tank.
Maybe I don't understand what you meant, but it's a bit misleading.