@malas_reefer sorry for your loss... in the old days when a fish would die over night or randomly mom and me would freeze the fish and take a water sample from aquarium to the store from where we got it before the time of internet
Test your parameters. Nitrates would be a good starting point
This is what i would do check all the parameters... another question is where was the fish sourced from it is recomended thst you qt a fish after purchase in cupramine and machalite along with praziquantal sometime annehyde too, when people get fish in the mail or from a dirty system for 5 weeks alot of people that run live sand also let the tank fallow(no live stock only rocks and water)to rid the water of any dormant ciliate(marine parasites that feed on fish)for 6 to 8 weeks usually consider it first cycle
But another question too is would your clownfish start to change color when the lights go out or before they turn on etc etc, then they pick an area of the tank they stay there,now clownfish are normaly teritorial but not like this, you notice your clown fish didnt eat today like normal, you see the gill plate start to flutter harder than normal then it starts gasping for air...somethings not right you notice your fish is rubbing up against rocks, getting blasted by powerhead got sucked up in filter,your fish is letharhic its getting dropsy it picks a part of the aquarium to hunker down in a corner or in a cave on sandbed, you notice little bubbles around fins tail mouth,your fish is starting to turn from its once vibrant colors to a pale dull shade of its former self... you see white spots all along the scales...by this time its to late your fish is suffering,getting eaten inside out by a parasite known as flukes 4 to 8 hours have gone by your fish is dead!getting smaked around by the flow sucked up in over flow or takung a sand nap
If this was your story then you may have had flukes dormant within the fish
Causing marine velvet, ich or whitespot disaese,if this is the case save your other live stock now get a hospital tank setup dose cupramine product machalite or annehyde aswell as peroxide qt for 8 weeks and let display tank fallow(no livestock)for 6 to 8 weeks to kill any dormant ciliate treat tank with peroxide (do 80%water change and dose 1ml of vodka then reitroduce fish after