Sometimes the salt contains nitrate as well, but:
The bacterial sheets (mulm) doesn't need to be removed. In KZ, that is what is dosed back to the system via ZEOvit reactor.
I recommend perservering thru, yes. It is possible as I said above, that 4 mL/day isn't enough, and I think your number will be around 7 mL/day. However, in the spirit of KZ, we'll ramp up this amount over a couple of weeks. During this time, nitrate may increase, though it should be less of an increase than before, if you are feeding the exact same amount.
I am really looking forward to seeing what the test results will be in 1 and 2 weeks.
Keep in mind, at the higher range of nitrate, precision is reduced, so it may not be obvious we're improving.