I believe the skimmer would be the first one to introduce pollutants to the water column obviously trough air, for this matter there's some prefilters sold for this porpoises, you attach them to the air inlet and replace the media as necessary.
All depends too in what pesticides the people who's applying them is using, there's some difference between them, narrow spectrum and braod spectrum, normally depends on what you ask your pesticide company to use around and inside your house depends on your needs (this would be neccesary to ask them about wich is safer for the tank) many factors include the use of them, kids in the house, household pets, and what your house has to be protected of, bugs, ants, rodents, etc etc.