Why my fish is turning black?

Question
Hi Chris, I was wondering why my lovely Pearlscale is quickly turning black (in about a week!). The tank has been set up for about four weeks now, it is a bowl with about 25-30 litres. There is a Blackmoor and a Bubble Eye sharing the bowl with the pearlscale in question. Had the PH checked by the shop a few days ago and they told me it was fine. The water has been changed every week and I have installed a filter in the bowl. The fish is  swimming happily and doesn't seem to be in any distress that I can tell. Just wondering why it is changing colour? I can send you a photo of what he looked like the day I brought him home, and what he looks like today if that helps. The difference is quite drastic. Love to know what's going on, and if I'm the bad mother here.... Regards, Isabella  

Answer
Hi Isabella;

He is suffering from ammonia and nitrite burns. The tank is too small for even one goldfish on a long term basis. Toxins from their waste are building up and burning their skin. Each goldfish needs at least 20 liters of water while small, and there must be a good filter. I'm so sorry you were misled by whoever sold them to you. Here is a website that tells about goldfish and their needs;

http://www.bellaonline.com/articles/art3503.asp

At Your Service;
Chris Robbins