Don't be too hard on yourself. If you are in the hobby long enough, you get to experience these episodes. Large amphids are tough though. On the bright side they are much less toxic that the others. Annoying, but not deadly to your SPS.
1) Falling ALK is typically a good sign. SPS are consuming it. It is commonly recommended not to do WC during dinos; that could also account for falling ALK as you are not replacing it.
2) Hanna or ICP is the only valid testing for PO4. None of us can tell the difference between 0 (starvation) and .1 (abundance). Make sure the cuvettes are absolutely spot/smudge free. I wear nitrile gloves.
3) Dosing nitrates LOWERS phosphates. Don't dose nitrates unless you are certain you have plenty of PO4. Seachem Flourish or NeoPhos are good premixes.
4) If you are PO4 zero in a system your size, it will require a SHOCKING amount of PO4 dosing to keep a steady value as it binds into the rock. Think liters, not milliliters. Just not all at one time.
5) I used a DIY PO4 solution with Trisodium Phosphate. Beware, there are MANY substitute products for TSP that are something else. I had to refill my doser 3 times on a similar size system for 1 month to suppress ostreopsis.
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