I can't say anything about other ones but I believe derasa clams are meant to be on the sandbed so I have mine on the sand. I have two derasas, a giant one that I got from lowsingles in feb this year and a small one from live aquaria a month? ago. https://imgur.com/a/uKsUuN1
I am running hydra 52HD, and the big one gets 180~150 par and the little one gets like 100~120 par. Running with a refugium and infrequent auto water changes, no skimmer and filtersocks. Nitrates 5~10, phosphates .09. Alkalinity is 7.11 being tested 4 times a day by alkatronic and dosing all for reef to keep things stable.
I dose kz sponge power at full dose and LPS amino at half dose manually, not dosing any phyto right now.
I'm surprised to hear you're having a lot of trouble with your system considering your tank photos I see. Maybe try dosing more clam foods like sponge powder?