You did not mention how old your aquarium is or how this problem occurred, that is, all of a sudden or gradually. Could be important to know before you start throwing solutions at the problem.
I calculated that your water change regime should have reduced your nitrates from 40 to 14 ppm in seven days if nitrates were being produced slowly. I assume from you message that you nitrates have not budged in a week.
Before you do anything, confirm you have a nitrate problem. You might have problems with your coral unrelated to nitrates. Get you water tested by another person, or with a new kit (I use
Salifert).
You did not mention phosphates which I would expect to be elevated. And you did not mention high rates of algae growth, red slime or the need to scraped the tank walls.
Nitrates accumulate because your feeding is exceeding you aquarium's capacity to remove them which also includes your maintenance on the tank. It will take some detective work to determine what part of your system is the weak link in denitrification system (to much food in vs not enough waste being transported out), but the suggestions by the forum to remove waste accumulation on the bottom of the sump and to remove sponges seem like prudent, cheap, low risk actions you can take today.
I would also consul setting a goal to lower nitrates over weeks rather than days.