I have a 55 gallon fish tank with two koi and originally four goldfish. I have maintained this aquarium for years very successfully, until just recently. About three weeks ago, I noticed that the smallest of the goldfish started to become very inactive. He would let himself sink to the bottom of the tank, usually behind plants or the undergravel filter tubes, and stay there all day. Rarely would he come up for food in the beginning, and towards the end he wouldn't come up at all. I constantly looked him over for swollen eyes, gills, discoloration of any sort, and found nothing. His color even remained bright and seemingly healthy-looking right until the end. Obviously, I speak in past-tense about him because he is dead. I found him caught in the vegetation this morning, still alive, but completely unresponsive. Should I have removed him from the tank a long time ago? What was the problem and could it travel to the other fish? The tank is very aerated and I am very careful about proper water going into my tank. What did I do wrong?