What a pity.
I suggest that you look up posts by Sterling, who was up, after dark, feeding the basketstar...EVERY night. Sterling actually DID have success with one so instead of listening to an LFS that wanted to make some MONEY, please check with people who learned, the hard way, about keeping them.
This animal should be BANNED from this hobby because keeping is SUCCESSFULLY means it will OUTGROW nearly any tank. THey get to be about a meter wide (3 feet) when fully opened, and should be left in the ocean where they can be appreciated fully.
They are a large, long lived animal that generally perish in this hobby because of the difficulty of care long term, the nutrient load on the tank from CONSTANT feeding, and their large size.
They are also generally nocturnal, and look like little more than a pile of string during the day.
2 days is nothing! Talk to me in a few years, and then you've done something. Another animal that will take a good year or more to starve to death. Go back to your LFS and ask how many have kept them for YEARS and he'll probably make something up.
Still, Basketstars (not usually featherstars) CAN be kept but require enormous effort and dedication on the part of the keeper...lots of research before purchase to understand and accept the responsibility. They are not a "just add some food" animal. Imagine just how much food needs to go into a tank to sustain an animal that is 3 feet across and only eats very small particles???? If kept, they belong only in a species tank.
Watch your water quality. If you are actually feeding it enough, you will probably have issues. And start saving up some $$ just to provide for it. Of course, I am sure your LFS can sell you a lot of food. LOL. But bottled "pour in a capful" stuff won't be suitable.
Another impulse purchase from the initial post but if you've done your research, what species is it? How do you know it eats phytoplankton?
If it helps,and if you have the common basketstar you have Astrophyton muricatum and a couple of people have been keeping them. Please look up how diverse the diets are that they feed (in addition to looking up posts by Sterling). Note very little phytoplankton is fed as they tend to eat larger plankton.