Sanders recommends 25mg for up to 85 gal, 50mg up to 170gal, 100mg up to 340gal, 200mg up to 680gal.......if you run the unit with out an air drier you get about 50% of the capacity of the unit........I have a 110gal tank the reason I got the 100mg was simply that the 50mg's were out of stock........I didn't want to wait two weeks