Did you ever consider the nutrient value of corn flakes or spam vs. fresh french bread and filet mignon?
I feed my fish commercially available frozen foods mostly because I am too lazy to prepare fresh fish, clams, shrimp that I've diced up (and then frozen into cubes) like I used to when I was younger. I don't drain them but I watch the P level using Hanna ULR Phosphorus.
I was having difficulty with higher levels of P (120 ppm) and the SPS coral growth was nearly zero while some LPS grew very quickly. I started using GFO in a canister filter, the media clumped and didn't tumble but the water had to pass through. I used a few tablespoons to start and it took a few weeks, changing media every week before the numbers in my 150 gallon system (water measured, not cubic ft est.) began to drop.
As someone mentioned earlier, the removal is working against the stored P that is resting on the substrates in your system as an equilibrium equation. Since you like numbers, thing about 10 stacks of 100 dollar bills, 10 bills deep. The first dose of GFO might remove the top 5 bills but you still have 10 stacks that are going to measure the same amount of precipitation as when you started. One more treatment and now you have no "savings" in the tank but the number still can read as high as you did before, but there is no reserve to bank against. Your next treatment is your last, and all your corals die from starvation but the number you measured is now finally low (at zero).
So don't start adding GFO while you are enjoying Scotch.

Do it carefully before and know what to expect.
With an eel in the tank you are going to have a real challenge keeping the eel fed and the P numbers managed, imo. I think you can do this. You like numbers and have raised two children solo. This should be a cakewalk.
One more thing about your Hanna ULR measurements; mine is old and I have to use a conversion table to correct to get phosphate estimates in PPM from the PPB phosphorus measurement given on the read out. .035?
Our 3 have grown, so I'm grabbing my medication now too.