id look into not only buying a protein skimmer (look for one rated somewhere between 180g-220g) but also adding another 60-75lbs of rock and also swapping out the canister for either a refugium or sump. (wet/dry or trickle may not be the best option) the fx5 is a GREAT canister filter and can handle a ton of water, but in a 120g sw tank its prob going to pushing it towards its limits, especially when you are fully stocked. thats another aspect as well. with your current setup, IMO you will max out alot quicker in terms of # of fish, as opposed to buying a decent skimmer and upgrading filtration