Jumbo Neon illness

Question
I bought 6 jumbo neons and when I got them home
I descovered one of them had a ash like coloration formed underneath his dorsal fin. He was not swimming right. He died that night. 2 days later one of the other neons developed the same thing. This one looked healthy at first.
now it stays to itself and hides in the folage away from the rest of them. They are 1 inch long.
Also are Jumbo neons given hormones to inhance color and growth.
will this illness affect my other fish.
I have a 55 gal tank with a  simplicity plus filtration system. I have 3 angels 1 pleco 7 danios and 1 yellow lab and 1 rainbow shark. Ammonia level 0 Nitrite 0 and nitrate 0. Its been up and running sence March 05. It finally completely cycled in May. I know what you are going to say 0 nitrates. And the answer is yes. I don't know how this is happening but my nitrates went from 5.0 to 0 in May and have not risen sence. I use plastic plants and ornaments. Ph 6.6 Temp 82F I do 50% water changes every 2 weeks. Water crystal clear.
I hope you can help.

Answer
Hi Silvrfox;

Low nitrates is common in new tanks. Your water changes are definately helping. You're doing a great job with it.

The neons may be infected with a parasite called "Pleistophora hyphessobryconis", commonly known as "neon tetra disease". It is not curable I'm afraid and the rest of your neons may succumb to it as well eventually. It appears to remain dormant in many fish until they are stressed from being moved from fish farm to wholsaler to retail store and eventually to your home. Keeping up your diligent maintenance and keeping the tank population low will help keep it at bay. Isolation of affected fish and sterilization of the main aquarium is the only way to prevent it's spread.

Here is more info on it;

http://freshaquarium.about.com/cs/disease/p/neondisease.htm
http://www.thetropicaltank.co.uk/hdntd.htm
http://www.aquariumpharm.com/disease/neondisease.html
http://www.fishprofiles.com/articles/general/microsporidian.asp

Wish I had better news......

At Your Service;
Chris Robbins