I was reading forums on how to keep water tanks clean and came across your suggestion here to put a hydrophobic vent filter on the tank vent line. That makes a lot of sense.
I've searched extensively for this product and it's hard to find. There's a whole class of vent filters for the lab market (Pall Acro 50, Millipore Aervent), but their filter size is way below the 1-2 ft^2 you recommended (20 cm^2). One claims 27L/min air flow, which seems marginal assuming say, a ~5 gpm fill if I've done my math right.
The only filters I've found larger seem to be aimed at massive tanks for the beverage industry and cost in the thousands. I'd consider up to about $100. Any suggestions for brand / model / where to buy?