Chances are that if it's a hydro/grow shop they already know...I mean there's a hydro shop by me that even has pics of pot plants in the windows! Just stick to the truth, except the weed part, easier to remember than a lie. Most likely they won't ask you what you're growing, just how big of an area you're using the fan for to help you select the right size. If they do ask what kind of plants say something like lavender that can smell quite a bit hence the need for a filter. I bought my fans at home depot in the bathroom ventilation section and they don't give a crap what you're using it for as long as you pay them for it lol.