Unless you see an anenome eat a fish I would not automatically assume that is the case. Especially a clown. BTAs in the wild don't normally catch whole fish. They don't get hungry in a few days. They can in fact go weeks and months without food as they are photosynthetic for the majority of their needs. Anenomes have some of the lowest dietary requirements of any animal. They are not little Audreys.
I have only seen one fish eaten by an anenome. It was a powder blue tang that was nibbling too close to a carpet and got nabbed. Carpet anenomes have very strong stings and are very bid. This one was almost 2 feet. Tangs are dumb around anenomes - they see algae and don't use common sense.
I had a flamehawk that was stung often by a long tentacled anenome (Macrodactyla doreensis). I had a tomato clown pair in the same tank. The clown used to harass him and run for cover in the anenome. When the hawk chased to close he would get tagged by the anenome. Then he would sit on a rock and twitch for a bit.
BTAs have very weak stings. Long tentacles have a much stronger sting and could not kill the hawk. I doubt a BTA they could kill a fish, and certainly not a good swimmer. The fish simply won't stay in the anenome long enough to be lunch.
There are many reason fish perish. Could be another predator in the tank you don't know about. Fish corpses also don't last for days. Pods, crabs, shrimps, all will devour the corpse in a day or two.
And clowns don't get eaten by anenomes. They bathe themselves in the anenome slime and the anenome can't tell the difference between the fish and themselves.
There are many more likely suspects for eating clowns.
