Time for a major rock and substrate cleaning! Although right now your PO4 is probably reading at or near 0, that is only because your GHA is using it all up. Take the rocks out and scrub them in a bucket of tank water. Rinse them in another bucket of tank water. I use a very stiff plastic brush for that job, Then, clean your sandbed. Use a Python Siphon or similar to remove the detritus that is in it.
Now, quit overfeeding and keep up on water changes. FWIW - I clean about 1/3 - 1/2 of the sandbed every water change. Doing it that way will allow the untouched portion to reseed the freshly cleaned side with microfauna lost to the cleaning process.
Although there are some creatures that might eat it (Sea Hare), they will just poop out the remains, adding to the nutrient problem.