Giant Clams
For many, many years out reach for marine aquarium keepers are the giant clams, Tridacna species. And to this day. though captive raised, giant clams are not for nascent keepers.
That said they can be kept if one has the intensity and quality of lighting, a steady temperature, and natural sea water levels of minerals, and sufficient alkalinity to buffer it.
The two most common reasons why giant clams fail in aquariums are insufficient light, and predation. In the former, clams contain a symbiotic algae species in their flesh. Like most all algae (other than cyanobacteria of course), the algae that give the vivid colors of giant clam flesh photosensitize light. The wastes from that photosynthesis are sugars that feed the clam. Also, giant clams need a brisk, chaotic flow, similar to that needed for Acropora species corals.