I had a similar thing occur to my F1 Anidnoacara stalsbergi last November... Grew him and his siblings out for .5" fry to 6+" females and 9" male.. One day he was sitting on the sand and I knew something was off.. I tired epsom salt, then medicated food, then GC and Kannaplex.. I even consulted a Fish Vet and had a virtual consultation w/ Dr, Basleer.. Nothing worked.. Was told I'd need to take him in for invasive examination to find the real cause. He never recovered, but is still alive.. I moved him and his 2 females to a pal.. we're both growing out F2s from my harem, so all was not lost, but still sucks...
I initially thought maybe he scarfed down some zucchini (for the plecos) too quickly.. A pal told me cichlids cannot digest zucchini skin.. But I figured he would have recovered over time if that was the cause.. My theory is he maybe bit off more driftwood than he could chew and it led to internal organ damage.. I have no clue, but just odd how he went from constantly patrolling the tank, to sitting on the sand.. water parameters were the same as yours except y Ph was 7.2. Fins were perfect, eyes clear, no indication of injury or disease... he was in a 120 w/ 2 females 8 Colombian tetras, 6 tiger SDs and a few plecos, and no one else showed any symptoms of distress.. Sucks.. Hopefully you have more success than I did..
