If the water in the lake is too clean to support the fish population successfully, it certainly isn't going to rot your skin or make you have two headed babies. Although it sounds like a great basis for a movie, Pod people of Daybreak.
My suggestion to cure the pay for non-resident blues, assign each home 5 resident use cards. That way the empty nesters can invite a couple grandkids, singles can invite friends, and families larger than 5 or people that want to invite a bunch of folks have to buy guest cards. Even Steven.