if the LFS didn't havea light on it then it was dying anyway so returning it to that store would just kill it anyways. I have read on multiple sites and a few white papers over the years that a dead anenome will potentially cause an ammonia spike (like anything else that is left to rot in the tank) but the cysts will only kill your inhabitants if they are allowed to get into the water column. This could happen from powerheads or pumps tearing them up, and I guess to an extent if there was something large enough to really try to eat it and rip it apart. We had an anemone that died while we were on vacation. I assume it was dead for approx 5 days (we had a house sitter that said she couldnt find it after the 3rd day, and it was a huge nem so it was easily seen if healthy). By the time we got home I had a trace reading of ammonia but I ended up finding it behind some rocks, delicately yanked it out, did a healthy water change, and all inhabitants were fine. I am yet to see/hear/experience first hand of a dead nem literally nuking an entire tank without "the rest of the story" having a viable explanation