Are you running a phosban reactor to deal with phosphate, etc. in the water? Many clams did not do well with them....since then I have my phosphate reactor in line with a reactor filled with carbon. Since I use Power UltraPhos by Fauna Marin it doesn't need to be changed as often as carbon, so I change the carbon twice to once of the Ultra Phos. My 2 clams are fine, one large and one small and I've had them for over a year.
Do a search on the Clam, Mollusk forum and you'll see a number of threads on it. Try "clams and GFO" or "clams and phosphate remover".
A clam dying that fast after acquiring it could have had problems to start with. But I have no doubt that the Flameback had a taste after it was dead and you will not be able to trust it again with a clam in the tank.
A slow acclimation and a good scrubbing of the outer shell are in order.
Also if you have Narrasius snails and did not put your clam onto a piece of rock for it to attach its "foot" to and it began to burrow down in the sand to find something solid, the snail may have eaten it's foot off. I have never had a problem with that since I place all my clams onto hollowed out pieces of rock so their foot is protected at all time, but I do know people who have placed clams on the sand only to find the snails having a feast. Also makes it easier to move them when I want and I don't damage them at all. Narrasius snails are a great clean up crew, but always hungry and a clam foot is just too good for them to pass up.
