There are two common treatments. Muriatic Acid and Lanthanum Chloride.
Muriatic acid will eat away at organic matter and the outer shell of the rock getting rid of any dead algae that was growing on it.
Lanthanum Chloride can also be used but its approach to fixing the problem is different. Rather than eating away at the rock and organic matter, LC will attach itself to the phoshate and form a precipitate and make the water look cloudy. This is the bonded phosphate that will be dumped away with the waste water. This process takes longer, costs more, and doesnt really destory the organic matter that could still break down after the LC treatment.
So today I went out and bought some muriatic acid and did a little cookout.
This batch I just started with my smaller pieces and rubble from when I cut off the bottoms of my main display rocks. I wanted to test the amount of dry off and crap that would come off the little pieces and make sure it wouldn't destroy the nice rocks I was planning to use.
A few minutes later you can really see the brown gunk that started to rise to the top.
When it stopped bubbling I emptied the solution and pressure washed off anything remaining. Tons of green and brown slim as well as numerous chunks of tiny organic debris was left on the bottom.
After I saw the results of the bright white rocks with no more crap I refilled the container, added the acid and dunked the large pieces. Lots of bubbles now!!
Overall tons of crap was removed that didn't need to go into the tank.