Smart Financial to start charging $15 monthly service fee

Received a letter from Smart Financial (the credit union, not the venue) that they will impose a $15 monthly service fee if I don't maintain a minimum $15K in my accounts. This seems rather extreme although maybe I have just been spoiled through the years of not having any service charges. Is this out of line or is it pretty normal to be hit with bank service charges for not maintaining a certain balance?