Sorry for the multiple posts, I tried to delete them to no avail. Clams need a lot of light….way more than sps corals, so the most likely problem is they are not getting enough light on the sand bed. Maxima clams need to be on a hard substrate but even on a hard substrate I couldn’t keep them alive long term. For me, clams require more care than sps corals. One of the best characteristics of clams is they are nutrient sponges and keep nitrates and phosphates down. When I get a bigger tank I will definitely get another one!
Darren