There are many contributing factors that will increase nitrates.
What filtration are you using?
What is your bio-load?
How much food are you giving them?
The replacement water for both top off and salt water...what are the trates in them, and have you tested them?
Do you have a protein skimmer? A refugium?
let us know so we can pinpoint.
Sometimes, water changes alone will NEVER do it. You have to seek some other options, by answering the above.
As far as water changes, it is fine to do what you have, but now take a break. You do not want to remove all of the nessecary bacteria etc that contributes to the bio-process.
Also, you seemd to do a few water changes in rapid succession...are you mixing up your salt water for 24 hrs. +? very important to do this for ionic balance, wich in turn can affect the bio=process