Starfish are not suited to home aquariums at all. A blue linkia starfish is for extreme experts, IMO. I would return to the store asap and get a serpent or brittle star if you want one.
I have kept one before. It starved to death. I have not seen a single case of them being kept long term in captivity. They are not suited for home aquaria as others have stated.
I saw a nice blue linkia at my local shop...... the guy said “I will sell it to you, but they die if you look at it wrong, and sometimes even when you don’t”