I'm not sure why. I lost 4 Zebra Danios, 1 swallow tail minnow and a B.A. Tetra. THe Danios I think died when I poured more water in the tank, from turbulence or something. Then I come back and find the minnow and the tetra floating around. I'm not to worried about it, they are relatively inexpensive fish. All my other fish are fine so I don't think I will find anymore floaters. I just added a power sponge filter for aeriation. Can fish die from too much oxygen???