I wonder if something else is out of order?
I have been seeing several posts about the micros in CSM+B perhaps being too high when dosed per EI levels. You might reduce this a bit, see what happens.
I dose EI, KNO3, GH buffer, Micros+B
Does this mean you are dosing KNO3, GH buffer and micros+B over and above the EI dosing levels?
What are the levels for all the tests you have in between dosing and water changes?
For example, right before and right after a water change?
Through the week the levels should be rising, reaching a peak right before a water change.
Then, given a 50% water change, the levels should drop by 50% (unless of course there are some things in the water you are using to refill).
I think the trace of phosphorus in the tap water is not a big deal, but if you want to reduce the phosphorus dose enough to compensate, that is OK.