Thanks,I had similar documented on here over a year ago with my WCMMs. All but one fish in a tank of approx 25 died and that was a hybrid Japanese Ricefish-Daisys Ricefish which may have been tougher than the rest who were almost certainly inbred to a degree.
I believe my tanks problems were a perfect storm of a psycho guppy biting other fish and me forgetting to use my “baffle container” when refilling tank via python which meant sand was blasted everywhere creating murky water. I think bacteria from the murky water infected the open wounds of a lot of fish and spread like wildfire through the tank taking over a month to finish them off despite daily huge water changes and salt treatment.
I saw bite marks on corpses and actually pictured him biting a WCMM while trying to photograph a diseased fish to show the pic on here.
I nearly threw the towel in again on fishkeeping and if that one Ricefish hybrid hadn’t survived I may well have.
Best of luck.
i've only got WCMM but the sand theory makes a lot of sense. Thinking of dark gravel next time instead, might reduce this issue.