I have had one for about 3 years M.Madox. It cam in as a hitch hiker. It was the base of one on my sps colonies. I didnt een know it was thier until I saw the colony tipping, I though the colony was loose but when investigating I saw the oyster, it was pretty big to, about 4 inches.
It died this september due to a temp and salintiy change when I re did my aquascaping.
They eat detritus and the bacteria associated with them, doc absorbsion and will take in phyto plankton and the zoo plankton that can be associated with them. In a hobby reef tank location is a key thing, try to locate a place where detritus tends to collect, or if you have a dsb then close to it also, athough I would not place it in the sand.
They are very delicate in regards to temp and salinity changes, so a very mature system is required. They also tend to have alot of external parasite (protozoans also) so make sure where ever you get them from that they are quarranted for a period of time.
Definately a tough species to keep.
Mike