Mysterious goldfish death

Question
I buyed a large oranda goldfish 3 months ago... he was one of my favourite... he had no disease and was active. I always did water changes and kept the tank clean. I did'nt overfeed them. One morning when I was feeding my goldfish, I noticed that my oranda goldfish is spinning horizontally like a tornado and eating its food like that. I immediatly became concerned thinking that if he has swimbladder disease.. after the feeding he became normal again and started to swim like normal. So I was relieved and thought that nothing happened to him. Later after 2 hours I noticed he was swimming abnormally, he was swiming upside down and sometimes normally.. I tried to remove him and put him in the quarentine tank but my elder brother insisted that nothing is wrong with him, and I agreed with him. Later we went out of the house and came back at evening, and I saw that my oranda goldfish is floating and dead... when I removed him from the aquarium I saw that his belly is green. I have 5 goldfish, 2 blackmoors, 1 comet and now after the death of my goldfish, I have another oranda goldfish. My question is:

Q1: All my other goldfish is healthy and active till today.. so why my oranda goldfish died??
Q2: What was the reason??
Q3: If it was any unknown disease that I don't know, please state how to cure it, so that I can cure my other goldfish if they have the same problem.
Q4: Why did he had green belly??

Answer
Hello Mafarah:  Sometimes goldfish get intestinal blockages and behave much the same way as you have described.  Sometimes though, it may be some natural phenomenon like a stroke or other genetic disorder.  I suspect your fish had a small blockage of food.  The green color on his stomach is because after the fish died his tissue became a little more clear and you could see the fish food inside of him.  I would feed them brine shrimp or tubi-worms at least twice per week to help avoiding blockages.  Your other fish should be fine and unless they start to show similar behaviors I would not worry.... dave