They disappear completely at night. My parameters are as follows:
pH 7.85-7.95
dKH 9
NO3 0.4 (down from 1.0 last week and 1.1 the week before)
PO4 0.06 (0.04, 0.03 in the preceding weeks)
Ca 410
Mg 1170 (added 570 mL of BRS liquid mag after that reading)