My zebra danio has always been active, and constantly swimming around for the last year and a half Ive had him. I have a 5 gallon tank - also in the tank is 1 cory. They have always gotten along - if anything, my danio would chase the cory. My ammonia and nitrite levels are 0, pH is 7.0. My cory seems perfectly healthy. ALl my danio does is float in the corner of the tank under the filter. It's almost as if he's scared or something. ANy idea what is wrong with him??? Thanks!