Hey i have the same tank as you! haha, um so it sounds like your tank wasnt fully cycled which could cause death, but also petsmart can have some pretty un healthy, and poor conditions animals which is just sad! I however only shop there (where im from in canada, they are much better than any LFS
From personal experience, i bought 3 albino corys i think from them, or some bottom feeder type thing, and they all died over night for no reason! so just some thought, if i were you id let the aquarium cycle for about 3 more weeks, just so BB is built up!
Oh in case you didnt know, to precerve bb dont rinse your bio-filter part except with tank water so it doesnt die! If you have any more questions dont be afraid to ask or PM me!