FWIW, After 25 years of trouble free as in never pumped, service I've had to start pumping as often as every month.  Two months ago I knew we were going to be gone for 6 days and I pumped it to just below the "normal" level a couple days before leaving, and added a bunch of Copper sulfate to the tank. It was about 18 inches over normal when we left. I had the person checking the house run tap water for 15 minutes each day. About 15 gal of water.  A month later the level dropped to normal and has stayed there through two onslaughts of grandkids and lots of showers and washing. I think I've dodged a bullet and will continue to use the CuSo4 at least semiannually.