Cannabis has few pests that we have to worry about indoors... mites often turn up when clones are traded between growers but a couple of sprays with Mite-Rid soons breaks their breeding cycle.
Fungus gnats will turn up in a sealed room regardless of how many filters are placed over the inlet vents, they will cruise in at ease when the human does the watering if they can smell stinky roots for dinner... no stinky roots = no food = no gnats, simple
White fly are probably the only pest that inlet filters seem to stop.
My view is that maintaining a healthy root zone and growing enviroment, as Louise touched on, is the simplest and most effective method for dealing with pests.
Totally agree with having a healthy root system, cause I was running a recirculating system I used an additive called oxy plus which is hydrogen peroxide (pretty sure) which cleaned all the dead cells off around the roots then turning the dead matter into oxygen, which goes into the water.