Protein skimmer functions.

A skimmer can not properly extract actual impurities!

They attract organic matter, water-soluble and particles and once that bubble has attracted organic matter it is useless for gas exchange and more.

The constant toxins produced by your corals, sponges and anything inorganic have nothing to do with those bubbles, once the oxidation processes and nitrate reduction are done, then your skimmers bubbles are near useless.

If they don’t grab it as organic then it remains.

From you need iron or aluminium and possibly carbon to try and take care of those and all of these are unreliable unless you pay a lot of attention to testing and timing.

With a skimmer it can only take it out if it gets to it before your nitrogen cyclers convert it to inorganic.
Then your skimmers bubbles are useless!
Now you need a water change.

A skimmer should be at the end of your external bio filter and aid your aquariums water quality and not be replied upon as much as many reefers do.