Sick goldfish? =(

Question
I have a 50 gallon fishtank which has been setup for about 3 weeks. Aside from 2 butterfly koi, I have about 15 goldfish in the tank (most of which are fairly small). I was away this past weekend for 2 days and when I came back, I noticed that my completely orange goldfish now has black patches on it. A few others are changing colors as well. Is that normal? In addition, I've also noticed that two of my larger goldfish seem to have open sores on their backs. I was wondering what may be the problem. Please help!

P.S. The goldfish with the sores on their back have been spending a far about of time at the top of the tank (taking in air?).  

Answer
Hi Sinlay;

You need to make a 50% water change right away and every day for awhile. I know it sounds extreme, but those fish are being burned by "New Tank Syndrome" toxins. That's what the black coloration is from. It's called "black smudge" and it's from toxin burns. You have so many fish packed in there so soon that the toxins have become way too high. Being up at the top is from their gills burning and lacking oxygen. Here is a link to my article on new tanks to help you understand more about what is going on in there;

http://www.xanga.com/Expert_Fish_Help

At Your Service;
Chris Robbins