Unexplained goldfish death

Question
I recently set up a 15 gallon tank with the intention of buying two small goldfish to live in it. I left it for three weeks with the filter and air pump on as I've been told you should. Before I bought the fish I checked pH, ammonia and nitrite levels and they were all fine. The two fish I bought seemed to have settled happily into their new home, but about a week later I noticed that one had started to swim around frantically and clamp her fins to her sides. On her tail she had what looked like a fish louse (I saw it move), so I checked the rest of her and the other fish and there were no others. The owner of a reputable fish shop near my house advised the best thing to do was to gently remove the louse manualy, which I did. A few days later the fish was still clamping her fins to her sides, and this morning (about 5 days after the louse was removed) I came down and she was dead. Why would this happen, and can I prevent my other fish being affected? Many thanks.

Answer
Hi Elizabeth;

Poor thing. It may have been from "New Tank Syndrome", or the fish louse caused an infection....or the stress of both just did the fish in. If the tank has been set up for less than 8 weeks, toxins will build up and make the fish sick if they get too high. Here is more about it;

http://www.xanga.com/Expert_Fish_Help

At Your Service;
Chris Robbins