[bertoni]

A 20% water change will reduce nitrate from 20 to 16, assuming the new saltwater measures zero. That level of change might not be detectable. The test kit might also be bad. I'd get a second opinion.

Mangroves and clams won't do much for nitrate reduction. Neither of them grow fast enough to accomplish anything much. Chaetomorpha needs to be growing rapidly, and then harvested regularly, to do much.

Is the flowerpot coral a Goniopora or similar species? They die quite regularly, so that might not mean much.

The feeding rate might be too high for the tank to sustain. What's the total amount and type of food going into the tank per day?