Considering how many people I have talked to that are told to do a water change to resolve trace element buildup after ICP tests, they are 100% still necessary.
The difference now is that advanced reefers that use good dosing including trace elements have an adequate skimmer, have an adequate refugium/ATS, use alternate means of nutrient export like carbon dosing / gfo, and regularly use carbon to prevent buildup of more toxic things can substantially reduce water changes.
If one relies on water changes for dosing and nutrient export, then water changes 10-20% a week is basically required. If all of the other steps are performed, I think 10-20% monthly is a good level.
The difference now is that advanced reefers that use good dosing including trace elements have an adequate skimmer, have an adequate refugium/ATS, use alternate means of nutrient export like carbon dosing / gfo, and regularly use carbon to prevent buildup of more toxic things can substantially reduce water changes.
If one relies on water changes for dosing and nutrient export, then water changes 10-20% a week is basically required. If all of the other steps are performed, I think 10-20% monthly is a good level.