That clownfish doesn’t look healthy. It looks like the scales and fins are not right. If it’s uck, they will look like little salt grains but your fish dying in such quick succession would probably lean more towards velvet/brook. If you keep putting new fish in a tank back to back that is contaminated chances are they are all getting the same disease from the previous fish since you already have one that is sick in the tank. Copper is tricky but you have to also get a copper test kit (a necessity) because the levels must be exact. You have to slowly build up dosage and then do water changes over specific timespans to decrease the dosage and it must remain constant for so long. At this point, Id say move to a quarantine tank because you shouldn’t have rocks/sand with copper in the tank (it will keep leaching it for some time after). Also depending on what your fish has, you may have to do multiple treatments to catch the entire lifecycle of whats affecting them. If you could get a picture of your affected fish or of the one that passed it would help as well to figure out what the fish has. Flukes or bacterial infections need things like prazipro and metronidazole. Velvet needs copper. Ick can sometimes be controlled by copper too in cases but each treatment has its own course times and parameters.