While its said that most nutrition comes to the clam from the products of photosynthesis, in the past when we sold huge numbers of clams and interacted daily with hobbyists dealing with those clams, the overwhelming feedback was the clams always did better in tanks that were not so pristine, in other words "dirtier" tanks with a well fed fish population. They do utilize dissolved organics. Once you see the mantle retraction in most instances that clam will die, despite numerous descriptions online for freshwater dips, etc.
Having been to these many locations and collected many clams, I can tell you it is rather difficult to cut the embedded clam from rock without damaging the foot. A hammer and chisel are needed and you must be held in place to do it due to intense currents. At least in my rxperience in the remote islands of French Polynesia, I needed a wet suit and felt like I needed a dry suit, the water was cold and crystal clear, with intense flow, and clams were never found below about 4-5 feet of the surface, usually just a foot or less under the intense tropical sunlight. In some areas there were Maxima clams in every direction for as far as I could see with no more than a few inches between them, in other areas there was maybe just a few on each distant rock outcropping.