I had three 3/4" clowns in a row die in my qt while all other fish thrived. The fish in the qt are all super passive fish like possum wrasse, whitespotted filefish and helfrichi of similar size.
The re-seller I purchased the clowns from did tank transfer method and they were eating .5mm pellets for him. Before I brought them home I matched my qt to his pH, salinity and temp and purchased .5mm pellets to have on hand. After drip acclimating them I introduced them to qt and they looked wonderful. The next morning the darwin was dead but the fancy looked great.
I tried feeding spirulina enriched brine the next evening and the clown had no interest at all so I tried the .5mm pellets and he wouldn't even look at them. There is no competition in the tank for food, everyone is very polite. I had business near the seller so he gave me a free replacement darwin and it too would not eat. I bought everything I could think of; pe mysis, cyclopese, bloodworms, ect.. The new darwin bit something two days later but spit it out. I assumed he swallowed a portion of it as I'd read the little guys will do that. Neither of them ever ate again and died about 10 days later.
I have an algae turf scrubber and sponge filter on my QT which keeps my water perfect. My water quality was also confirmed by lfs testing. I have no idea what happened or why the other fish which are all diversden purchases have had no issues.
Maybe the smaller clowns can do ok with a move or two but I am three hours away from the seller and it might just be too far for them?