My Feeder Fish is Turning Black!

Question
Hello Jaymie,
I have a small (not much more than an inch big) orange and white "feeder goldfish" (he cost me a quarter) that I purchased from a PetsMart about 5 months or so ago. Up until Black Friday, Meta (that's his name) lived in a basic fishbowl on my desk. On Black Friday, my cat knocked over the bowl! Amazingly, Meta survived, and I have since bought him a 2.5 gallon tank (with a LID) and added some real plants to it to keep the water well-oxygenated. He is a very active fish; he gets very excited when I come into my room or open the lid to feed him (I feed him every evening), and is showing no signs of harm from the near-death experience he had last week. However, I started noticing black spots o his tail, one fin, and along his left side. I know about tail rot, and checked his fins and tail for signs of it, but I didn't see anything wrong with them. He is as energetic as ever, loving his bigger tank, and eating as much as he always does--do you have any suggestions? It almost looks like he is bruised from the fall (He was on the floor when I walked into my room, and my desk is a good 4 feet or so off the ground).
Thanks,
Miri

Answer
Miri,
Unfortunately, 1 goldfish should not be in a tank smaller than a 20 gallon with a filter for a tank no smaller that a 30 gallon tank. Your "feeder" fish is also known as a comet. They can grow 12 to 18 inches. Being in the 2.5 gallon tank, your fish's growth will stunt. The skeletal part will stop growing but the internal organs will not. This makes for a very slow and painful death.  Chances of survival without a filter are very low. In that small of a tank, you will have to do daily water changes instead of weekly. What you are seeing now is most likely from the fall. 4 feet is a long way to go for a small fish. My suggestion is to get the right size tank for the fish, 20 gallon with the filter for a larger tank. Do daily water changes until you get the new tank and cycle the filter, and add some stress coat to the water as per the instructions. Once you have done this I think he will very happy to be in a larger home with a filter.