I'm just reading this and it was not mentioned unless I missed it but are you using straight tap and do you add conditioner? If your water source has high levels of chlorine/chloramine...large w/c's at 80% might have a slight affect the BB and more so if your squeezing out sponge media at the same time with tap water.
Just covering the common errors...but if all above checks out and does not apply then I would agree with the others saying that your over thinking this and would not stress out.
...Ralph