Well Nick thats a question with a long tale, lol. For me P is the amin battle i fight, I am not to worried about nirtogen as thier are many checks in place. Know that one must introduce P into the tank in order for it to be thier aides in the battle. Yes as Curt says you will never eliminate it but in reallity you never want to as it is an important element present in all forms of life. What we are looking for is the line that seperates normal and excess. Excess can be determined by an ammount of inorganic P (PO4).
Foods are usually the way most enter the tank
Salt mixes will either have P or N in them in excess, so I avoid the wones with P (tropic Marin is a biggie for P)
Addivtives are another big one, most additive labeld as bio mean they use sugars as carriers, and in most cases contain large ammounts of P. Also the use of CC or argonite sand in reactors release alot.
Allowing a sand bed to go anaerobic will free up a ton of bound up P in the sand and make it available in the tank.
Ok so thier are a few things you can do to control input.
Now on controling the export.
In our tanks your going to find P in a few places.
> first place is going to be bound up in sand or LR. For this I try to keep both of these well aerobic, this keeps the ph up and does not allow for it to be unbound if if it does it is quickly rebound. This is a tough battle with deeper sand beds though.
>P is always going to be assocated with food and waste. So feeding only what the fish will eat and then makeing sure that checks are inplace to remove the excess is critical. So all detritus and excess for must be removed as best as you can. Skimmers running wet, occasional filter sock and so on are very effective in remove this excess.
>next would be the exports of critters that bind P with in their matrix. Bacteria will envelop and multiply detritus and waste, they will also create a biofilm of organics that they use to reduce the above,these organics are loaded with P also, so in getting rid of the extra food, waste and detritus in general we are exporting both organic and inorganic P, best tool for battling it.
> last way is the excess inorganic P that is available in the water column. Once you system reaches a point where it can no longer bind up any organic P, P will then remain available in the form of PO4, this is what our test kits can test for. So if you have a reading of PO4 you know your tank is saturated organically with P. For this group I use a P remover such as rowa or phosan. any water that passes through it will have the PO4 removed from the system.
hope it helps
Mike