Some charcoal is loaded with phosphates. Check the packaging. A faulty or old RO
system can also add unwanted organics.
The live rock will leach P04 if there is die off, but a three year old tank, that should be over with. It's excess organic material, not just P04 that is driving the GHA. Your right, your readings are low because the algae binds the P04 before it can show on a test kit. A filter medium like GFO
, will compete with the algae and eventually win as it also is an effective binding agent for phosphates. Everything happens suddenly in a small system, so work slowly and make any changes gradually.
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