I have had my 37 gallon tank setup for around 8 months now without any problems. I have 6 Danio Zebras, 3 Cory Catfish, and 6 Neon Headlight fish. 5 of my Zebras have grown to about 2 inches long, but the 6th has not grown at all and is now not looking healthy. He or She has a shrunken...