If you mix the salt while it is cold over night, that will prevent the build up. Then turn the heater on when its all mixed up.

Are you trying to raise your alkalinity levels up a little higher? 6dKH makes me cringe.

Based on your tests, you don't have a parameter issue. Are you vacuuming the sand bed at water change? If yes, maybe ease on it for a while until everything gets settled down. maybe gas bubbles are being released.

Or feed more to increase the nutrients. The frogspawn will like it.