I'm not sure what's going on in my tank. I bought three pink danios on sunday, and on Monday two were dead. I went back to the store last night and bought one more and it was dead as well as the last one I bought on sunday when I get home from work. I'm not sure what's happening. I had the water tested at the store yesterday and it's fine. The temp is at 78, and the guppies are doing great. Any ideas?