First off, sorry about the first clam. Clams can be tricky and finicky and just plain frustrating. They die fast and sometimes without us knowing why.
Now, with that said, I love clams. If you are planning on getting a maxima that large, it will be wild caught or from another reef keeper. The captive bred clams are much smaller when up for sale (1-2"). So, you deal with other issues with wild caught.
Wild caught, you will want to inspect it really well for pyramid snails. Not just once, but frequently (every week or so for the first few months). You also will need to watch for any signs of minched mantle. Orion has some amazing write ups about this.
Next, make sure you have very intense lighting for these guys. Good flow, but not linear straight at them. Turbulent or random is better.
Once you get it and inspect and place it (somewhere secure on rock), leave it be. They hate being bothered much.
Good luck! If you want to try a clam that is a bit more forgiving, try a derasa or squammie. They grow fast though. You will need to monitor calcium/Alk/Mg for those guys.
