NKT:
For the T. max. clam, I will tell her that it is a:
1, biological nitrate reducer (denitrifier). This unique denit. take advantage of the stray light in the tank, and use the photosynthesis process to absorb nitrate in the water. Less water change is needed (25 water change per year instead of 26).
2, powerhead. It helps water circulation in your tank as water is suck in and push out of the siphons. If you add a water wheel to its output siphon, you can convert the clam to drive the wheel without electricity! It also doubles up as a water filter.
3, fish food. If, by chance, it dies, it will be a nice food for your hermit crabs and other critters.
3, decoration. This is obviously true when alive. However, when dead, the empty shell will be useful for mounting coral frag, or simply as a soap holder near your sink. Or you can use it as a scroop, or as a dish to hold seafood (minced shrimp, fish, etc.) to be baked in oven.
I am sure the above is worth $500 already, so you saved $300 by buying the clam.