ya sorry to tell you, but i feel the store ripped ya off for a quick sale..when cycling their are high amount of toxicns released throughout your tank. That is probably why your cardinals died..i dont know what to tell you, you could probably try gettin at the store for selling you false information. talk to a manager or something. im not sure if it will do anything, but sometimes if an owner/manager realizes what had happened, he might give you store credit or take the fish back and hold them for you, something, anything if possible, but just feel free to post a thread on here, there are very knowledgeable members here. good luck with your aquarium