Just following on the thread, I thought I could contribute some useful info...
Check this: I'm in the Philippines and I wild-caught my stars.
- Starfish should definitely not be exposed to air (like fromia and linkia) but serpentine stars/brittle stars/sand sifting stars, like the one in the YT vid, live in tide pools. The are exposed to air every day when the tide goes out. So exposing them to air is not an issue. There are so many stars on the rocks that in some places, it's almost impossible to put your food down without stepping on one.
- They are extremely hardy and tolerant of salinity changes, temperature changes, and water quality swings. Yes, because when it rains on the tide pool? How about the typhoon we had last week? They don't all die, right? And the sun is very hot here, at the equator and the little water that is in the tide pool gets very hot but still, they don't die. Drip acclimating them for 45 mins is perfect.
- Your sandbed depth is okay but could be a little deeper or maybe a little more evenly spread. (but it's okay as is) The only comment I would make is that your sand might need to be a little coarser at the bottom. I saw your flow in your tank and I think it's fine. My opinion would be you try another serpentine star. For mine, I have a palm sized live rock sitting on the surface of the sandbed. The stars park themselves between the underside of the rock and the surface of the sandbed and do not really move around much from under the protection of the rock, they basically stay there with their body under the rock and their tentacles out in the open to feed. If you were expecting them to roam around underneath the sandbed all through your substrate, they won't do that. They need a rock. I saw you have more than enough live rock in your tank as potential homes but for the serpentine stars to make a home, the rock can't be directly on the glass. They need to be able to sandwich themselves between the underside of the rock and the surface of the sand.
- As I understand the serpentine stars feed in two ways. They feed on very small particles moving in the water column, which they catch by waving their arms. They can also eat/scavenge pieces of food. So the way I manage is I have a rock (or coral) that is on the sand that they live under. I took only adults, slightly smaller then the size of my hand. This way, know they're there because I can see their arms sticking out. They pretty much react like nassarius snails in that they react when they smell food fed in the tank. That reaction will tell you that they are healthy and eating. I target feed every other day or every third day. So set up a place for them on the sand and they will keep that area clean.
I have one in each corner of the tank.
I also think it' s not your fault that your star died. Nothing to do with your care. It was probably already at death's door when you got it, so don't worry about that.
If you have more questions just ask and I will try to answer if I know.