Water parameters are good nitrates a lil high 10-20 test with api master. Already killed srt and another flowerhorn . Nothing on other fish in the tank im at a loss
Could be lymphocystis. Espnif stressed from other fish. Would add some salt 1 tsp per g and keep the water clean. Post ur water parameters when able.
Looks like this this morning did a 50% water change last night added salt any thing else would be very helpful