GSP Died Suddenly

Hello,

I acquired a green spotted puffer about six weeks ago due to an ordering mistake at my work ( I work in a pet store). Last night I cam home at 5:30pm from work and I found my GSP in the back of the tank at the bottom breathing very heavily. He was very dark, a ring of black went around this belly. He was green and purple looking instead of the yellow and black. He looked almost swollen to me to, which was odd because he hadn't been fed yet that day but his tummy was a little puffed. He wouldn't eat frozen blood worms when I tried to feed him. He died within an hour. The breathing slowed during the hour and became shallow, he couldn't swim very well and when he did it was erratic and he would bang into things. I had him in a brand new cycled tank ( I moved him from one that was much to small for him), he was in the tank for about 6 days. He was 2 1/2 inches when he died. Can anyone think of what caused his death? I checked the fish diagnosis book at my work today and the symptoms seemed to fit with either internal parasites or cysts.

It was a 10 gallon, (he was going to be moved into a 30 later all for himself). There were no other fish in the tank with him. I set the tank up (water was brackish) and let it run for 2 weeks before I transferred him into it. As for the parameters I can tell you the ph is at a good level but I've got nothing for the ammonia, nitrite, and nitrates. I've been digging for information on gsp's since I got him, I went to 2 local fish stores and neither of them mentioned how intolerant they were of the ammonia, nitrite, and nitrate. So I didn't even think to get the tester kits for them. Which makes me even sadder now if that's what killed him.

I'm also taking him to one of the local fish stores since they are almost exclusively brackish and marine ( i still have his body in a cup of water in my fridge) to see if they can't help me figure out what was wrong with him.

When he was dying, I tried changing a bit of the water and it seemed to p erk him up for 10 minutes and then things got really bad from there. I use Amquel+ and NovAqua as my water conditioners (if that makes any difference). Also, is it possible for a water conditioner to go bad?

I'm sorry, what exactly is the right way to cycle a tank? Because apparently I'm missing something, is there bacteria I need in the water? I thought that's what setting it up and cycling the water did. It allowed the good bacteria to grow in the rock bed. I have large pieces of slate rock in the tank from the tank he had been in previously. I put these rocks in when I began the cycling process and then waited two weeks. I saw in the links that I needed a source of ammonia to start bacteria growth, but I had the bacteria I needed from the rocks. So what am I missing here? Also, I work in a pet store, not a fish store. I have to help people with the general animal range (dogs, cats, small animals, reptiles, and fish).