The main reason a clam needs to stand up is light. That is their main source of energy and food. They absorb (if you will) light through their mantle. So if all of the mantle is not fully exposed to light the clam is not getting as much light as it could. Make sure your support is not going to prevent the clam from opening and closing it's shell. A clam may live for years on it's side but may be more healthy if it were allowed to receive more light. Also the bottom of the clam where the "foot" is, is a weak spot that predators can enter - like pyamid snail or even crabs could make a snack out of the flesh, thus killing the clam. Also maximas require alot of light - what kind of lights are you running? Most people put the maximas on the rock work because they prefer to attach themselves to rock and because that gets them higher up in the tank thus getting more intense light. Is it a blue, green or golden? If it is a golden then it requires less light than a blue or green but still it will love light. Remember it is frustrating when a clam moves around but they are doing this because for some reason they are not happy (ie. not enough light, too much flow etc). Usually (not always) they will wind up dead if they are not happy.
hope this helps - would love to see pics if you get a chance
