I'm currently batting one for two. I started with a 2 inch ORA derasa since I wasn't confident my water quality was good enough for the more opulent maxima or squamosa I really wanted. It seemed to do well for a few months, no obvious growth but seemingly healthy, but then it suddenly died within a period of days. Not entirely discouraged, I tried again with a 3 inch Hippopus. Six months down the road, so far so good (I think). Like the derasa, I see no new shell growth, but it's alive and shows no overt signs of unhappiness. I'm keeping nitrates to 5 ppm or below. I change out phosphate media if they go above 0.05. dKH around 10.2+/-0.5, calcium
>385 (supplemented daily so I don't quite understand why I can't get it higher), pH 8.0, temp 76 F, use Reef Crystals, and cast-feed OceanMagik (Algae Barn) daily. But my set up is pretty spartan - no automated dosing, just LED lights (my old metal halides were too hot for the house), and I have no tools to measure PAR or ORP. Still, my SPS, LPS, and gorgonians (also all probably new to my DT within the past year or so) all seem healthy if not growing explosively.
derasa probably within days of demise
Hippopus - Not a Tridacna by any stretch but one step at a time
Photobomb