The smaller water changes are not very effective. You need to just do one large water change ( I have no fear doing near 100% with adjusted seawater)... or a couple beefy ones (two 50% waterchanges in a few days).
Change carbon weekly, and Ozone is fab if available.
This all could have been avoided by properly quarantining the new livestock my friend. Hard lessons
Please do learn and apply proper QT procedure and you will go much farther in this hobby, my friend (and save organisms lives/not wasted).
Anthony