Read your new updated
profile. A "few" months old may be too few for such a substantial bio-load IMO
. I'm thinking that could be the source of your nitrates. Need to watch your fish closely at this point. New tanks, lots of fish, and not-to-good water quality can lead to disease. Got a QT
if treatment is needed? Maybe nitrates @ 20 isn't critical yet, but it could get worse, and since the inverts are dropping, sounds like you've definitely got to get a handle on it? Watch the feeding amounts too. Maybe only once a day?
Believe me, I found out the hard way about adding too may fish too soon.