Nitrates in Salt?

Doing too many water changes will disrupt your bacterial colonies in the water. Besides that, doing water changes does not solve the problem, it just temporarily removes the effects of a problem. Even at 50%, your 40ppm NO3 will be dropped to 20ppm for a short time before they reaccumulate back to 40ppm. Bio-balls can be a heavy source of NO3 as well. But when you remove them, remove them a chunk at a time, not all at once, or you will disturb the aerobic bacteria colonies that break down the NH3 and NO2. While doing the WC, you are also removing some of the anaerobic bacteria in the water that breaks down the NO3 into nitrogen gas, so it's a catch 22 in doing water changes. Personally, I only do about 10% a month now, and that is only to replace the used trace elements in the water.