Firstly, how long did you aclimatise the starfish before placing it in the tank?
MAny people say that a starfish should not be exposed to air as they can get airpockets trapped inside them and this will kill them (i have seen evidence that points to this being untrue however).
Starfish are extremely sensative to changes in water quality, even a slight change in SG or PH will be enough to do serious harm to a starfish. Whilst it is possible for startfish to regenrate limbs etc, its als much more likely that they are indeed dieing. I had a blue linka and a red starfish. both died fromthe same syptoms. They started with simple splits in their skin, this eroded them rapidly (the blue disintergrated within a day) until they fall to bits. If this happens then you must get them out of tha tank fast! they will pollute the tank and within a small amount of water, its likely that the starfish will kill everything alone with it.
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