Well, I have no exact figures in hand, but I will tell you that in my closed system with anywhere from 15 to 17 adult and juvenile clams, my nitrates are depleted rapidly. I am also constantly battling to keep my phosphates up. This is my biggest problem. I test my water every 3 days or so, and always get an undetectable reading of phosphate.
This is, of course, undesirable for clams. I have a heavy fish load to try and compensate for this, and I sometimes turn off my skimmer.
With a large number of clams you will notice a marked decrease in "measurable" elements and nutrients. With one or two in a large system I doubt they would make a significant, testable difference in water quality.
I have about 40 clams on my 150 g tank. They consume about 1-2 mg/l (ppm) of nitrate a day. If I didn't have a high fish load and I didn't feed my tank heavily I think they would be consuming way more nitrate. I add Sodium Nitrate to my tank every week after doing my water changes (10% to keep my Ionic Balance in line). I usually try to keep my Nitrate level around 5 mg/l(ppm). I usually check my nitrate level twice a week to make sure they don't get depleted. For people that don't want to add Sodium Nitrate to their tank just overfeed your fish and add more Phytoplankton(Live, Frozen or Dried).
However, it is very important to measure your nitrate every other day just to make sure you are not overfeeding or under feeding your tank.
Another observation that I made is that when your tank is depleted with Nitrate your clams usually fade or even on the extreme case they bleach.
Most of this info is written in D Knop's Clam Book ( The Bible of Clam Keeping). Remember as well that this book was written about 8 years ago and more research has been done ever since. One example would be feeding. As we know now that clams should be feed phytoplankton regardless of what species they are especially when they are younger (general rule would anything below 3 inches should be feed heavily).
I also read an article in here about a facility in Australia using clams as filters (Nutrient Export). I think Dr. Mac has one tank being filtered by clams as well.
Are you farming them, or just really keen on clams? Or a store? Are you Luke, the owner of Oceans alive in Toronto?
My clam moved a bit last night and seems to be just holding on to the edge of the rock with his foot, instead of resting on it. Is that ok?
(Also I noticed that their colours change depending on the angle you look at them at. Neat.)
I'm not farming them, I just really love keeping them.
I don't own Oceans Alive. However the owner knows me. Next time you drop by ask him if you could see a picture of Marte's tank.
As far as your clam is corncern, I guess it does not like its current position. when a clams is not happy at a certain location it will actually release its byssal thread and relocate itself or fall off.