IMO if you do not have metal halide lights there is very little (and I mean VERY little) chance of it living long term. But for now, feed it meaty foods like mysis shrimp, clam, squid, cocktail shrimp, etc. 3 times a week. Even if you do have metal halides, you should still feed 3 times a week.
Also a word about saving things from a LFS.
If you decide to save something, they would just buy even more to supply the demand for the animal. It is best not to save a dieing animal, even if it does die they should learn those don't sell and won't buy anymore.