I guess I should explain what I'm trying to accomplish. My nitrate are under control. They are around 9 ppm but my PO4 is high around 0.24 ppm. I'm not sure why the PO4 is that high. I'm trying to get it down but also trying to figure out why it is so high. I'm wondering if I need to use GFO or not. I would rather try and figure out why it is high but also know I need to get the PO4 down. I believe that if I can do that then my NO3 will drop to undectable. I'm I correct in my thinking? Do I need a reactor?
Running
75g
HOB reef octopus skimmer
Eheim 350 external canister filter
50 to 60 lbs live rock
Tank has been cycled for about a year now
When you put food in your tank you are adding phosphates and they will remain in the system and accumulate unless removed. Some small amounts will be used (but not removed) by growing things in the tank but unless those things are removed from the tank those quantities remain in the system... Nitrate on the other hand will gradually be broken down into its component oxygen and nitrogen in anoxic regions of live rock, sand bed , sulphur reactors etc. So that level is controlled by the balance between how fast your bio-load creates nitrate vs how fast it is broken down...and of course small amounts are exported with water changes. Nitrate can also be exported by growing algae in a scrubber and harvesting it regularly or providing a carbon source to grow bacteria and skimming them out. These processes will remove phosphate as well but only very tiny amounts compared to nitrate.
So bottom line is phosphate will accumulate until it is removed chemically by adsorption by GFO etc or precipitation into an inert solid with Lanthanum Chloride. Also as phosphate levels rise the rock and substrate will absorb it as well and constantly release it back into the system as levels drop, so if your levels are rising it means that your rock and substrate are likely getting pretty saturated...so it will take time to remove the phosphate not only from the water column but also the buildup in the rocks and substrate.
GFO is probably the easiest, safest option for small and mid sized systems...Lanthanum is far less expensive but can be more difficult to use effectively but is an excellent option for larger systems where the costs involved in using GFO can be significant...