the phosphates are bound by the bacteria. when the bacteria are done processing it it falls to the bottom of the tank. at this point if you have a substrate it is then absorbed by the calcium carbonate in the substrate. here it will sit untill the substrate is replaced. if LR is touching the substrate than the phosphates can be wicked up into the LR itself.
it is not untill the substrate and the LR are completely full of phosphates that they will start to leach back out into the water column. at this point you will start to get reading from your
phosphate test kit. by this time you will have all kinds of cyano and HA problems.
those bubbles are most likely nitrogen bubbles from the
nitrogen cycle being trapped by the cyano. the cyano will create a anoxic zone under itself where nitrogen can be processed.
if the skimmer can remove the vitamins and other trace elements than why put it in.

water changes will keep anything that is considered trace in check.
this hobby is expensive enough. you do not need to spending more money on suppliments and stuff than necessary.
G~