ETOH or Acetic acid dosing will NOT remove PO4.
Their function is to add reduced carbon(think carbohydrates) to the spartan feeding and loading that many reef folks do so that the bacteria can increase the rates of NO3 denitrification and some other cycles. Essentially the water is so pure that the bacteria become carbon limited. So adding a little bit of reduced carbon, it can come from most any source such as macro algae, it a wise thing to add.
If you spot treat, acetic acid will pickle any macro algae or any other critters you have that comes in contact with it at high concentrations and low flow.
So will ETOH.
So you can also use it to deal with some problem algae also.
Generally, if thr macros do very well, then there's little issue with other algae. Macros also provide a nice habitat for various other biota, supply O2, remove both NH4, and NO3, and PO4 also.
So no need to dose ETOH/Acetic acid, GFO's, Skimmers, etc.
It's also self replicating and you can sell the seaweed, feed it to your tang etc. It makes for a much more natural biocontrol method for nutrient management.
One thing about refuges: make sure to prune excess growth routinely, do not wait till the fuge is overflowing, then remove 95% of the biomass. Wait till there's about 75% full, then remove 25%, try to keep roughly 40 to 50% surface area coverage at minimum, but do not let it get choked. If the macro biomass is stable, then so are the tank's conditions and loading rates. If you can do that, then it really is one of the simplest and cheapest ways to filter the water.
This way the NO3/PO4 do not bottom out like it might if you had 100% full refuge, and not rise if you only had 10% biomass left AFTER harvesting it.
It's common sense if you think about it.
