In a pond! You should see them in nature when they are stuck in giant coral heads! If you think they have vibrant colors from above, try and look at one through a mask and swimming around it.
You could also construct a "look-down" tank like Gary used to make over at Harbor Aquatics and hang a MH pendant to light it. Marine Ecosystems has one of Gary's "look-down" tanks (dims 4'long x 2' wide x 1' tall) lit with a halide/diamond reflector combo altho it doesn't currently have clams in it. Joy's got some nice look-down tanks in her shop...here's a couple pix of the tanks in question (they're the clear acrylic tanks):