Wow sounds like you got your bases covered. Hmm...
How long have you used the dip stick test kit? They can be somewhat unreliable. Any chance you could take your water to the lfs
for a test, or get a liquid test?
I know this is not helping much right now, but that's all I can figure at this point.
PS. You sound like your doing this right, but I gotta ask. How much do you feed? Any chance you are overfeeding? Are you dosing any zoo plex ect... for the inverts and corals?