Reduck
Because I can....
you both seem to be doing at least 10 gallons every 2 weeks. I have been doing 5 gallons. Maybe it's just not enough of a water change?
10g is about 25% of tank volume. depends on the volume of rock work and sand and live stock requirements.. what works for us, may or may not work for you.
if your tests and tank look good at 5, why change?