They are not particularly difficult creatures to keep, but you almost need a species tank for them.
First, they are mud anemones. They live in silty, sandy bottoms where they build a mucus tube to live in. They withdraw into the tube when threatened. This tube can get pretty large and nasty; as it gets a half inch thick or more and it is kinda slimy. They are not happy in a standard reef tank unless it has a suitable deep (and soft) bottom that it can dig into.
Once settled, they are easy to care for. They are not photosynthetic, and will eat more or less anything that contacts their tentacles. They do better with small pieces of food (almost detritus) than large chunks. If fed regularly they can get very large very quickly. They will sting anything they contact, but I don't think their sting is nearly as powerful as that of a large anemone - or perhaps because the tentacles are so small and threadlike there just aren't as many nematocysts available to sting.
Once established they stay put. If they cannot find a suitable deep sand environment, they will continue to move by backing away with their foot/column (they move backwards). This may include abandoning an old mucus tube and establishing a new one. If this occurs, I recommend you remove the old mucus tube because it will be a rather large chunk of nastiness that you won't want in your tank. It you try to place them in a rock pile, they will back into the rock pile until they reach the bottom and then try to dig into the substrate. This may end up with you having a cerianthus anemone deep in your rocks.
They prefer moderate current.
Aside from that, there is really not much else to say. They will obviously not host anemone fish, but I did not find them particularly dangerous to fish, in general (as compared to carpet anemones for example).
