Yesterday I had a cory die from a unknown reason. Now today another has past on. I have a 10 gallon tank with (now after the 2 deaths) 1 Bandit Cory and 1 Panda Cory and then 4 Ghost Shrimp. The first one like I said died I don't know how but just past on, but the second one was different. He developed a red spot like a circle around his left gill only and was acting lazy. He was lazy when I got him from Petsmart after I bought him but did not have a red mark. I figured he was a littler stress from the 30 min drive. So I put him in and the next day he developed that red spot and only a few hours later he past on. What did he have? Was it a parasite or disease that he got from petsmart or was he just to stressed. I tested the water 3 times over the last 2 days and Ammonia, and Nitrite are both at 0 so I know it is not Ammonia poising. Any help would be great. 
