To begin running your saltwater aquarium without water changes you need to understand your systems requirements first.
Its safe to say that if you are not at the stage where dosing has become essential, and regular water testing has not become a religion, then having perfect parameters is not something you are trying to achieve just yet.
If however, you are auto dosing to meet your coral requirement, have robust phosphate and nitrate export methods in place such as GFO in a reactor, or using Red Sea NOPOX, then you are in a position to stop. The next step is to make sure that you have all the systems in place prior quitting the regular changes.
Without changing water you will need to provide not only KH, Ca and Mg (which you are probably already auto dosing), but trace elements too. I manually dose my trace elements every evening and send my water off for analysis with Triton every three months to ensure all is okay.
You will no longer be thinking about removing excess nutrients with water changes. Make sure you change out your phosphate remover like clockwork, and/or try not to miss doses. I use NOPOX every evening, and also keep my phosphate down with a product called Rowaphos which I change out every 4 weeks regardless of test kit results.
As a bit of a disclaimer – the above is how I run my tank successfully. You may have an aquarium deficient in nitrate or phosphate that needs nutrient dosing. The last thing you would want to do is put nutrient export procedures in place for a nutrient deficient system.
If you are interested in nitrate dosing click here
For phosphate dosing click here
Dosing of either of the above is safe. My frag tank is phosphate deficient into which a dose of 5ml of Tri-sodium phosphate is added daily.