You mention you do not add anything to your water. I just want to clarify, no water conditioners at all? What is your water source? If it's regular tap water, you need to use a water conditioner to clear up the chlorine and other additives most cities put in the water. Without a good water conditioner, you're fish can get very sick. I would strongly recommend looking at getting Prime, it is in my opinion, the best water conditioner out there.
Also, it looks like you may have had a bump in your cycle; since you have Nitrites showing. Ideally, you want your water parameters to read: Ammonia-0, Nitrite-0, Nitrate- less than 40. What type of tests are you using? I've had this happen with an Oranda years ago, and the cause of it stemmed from the water quality. Once my nitrates went above 20 he always flipped over. I would recommend doing daily partial water changes until your cycle straigtens out.
You mentioned that you feed them pellets, what kind? Floating or sinking? If they're floating, your fish may be swallowing air, as previously suggested. You said you've fasted him for 4 days, yet he is still floating. Have you fed him since then? I would recommend feeding him peas after fasting, that way it will clean out his digestive system.

My Oranda that floated would remain upright if fed fresh fruits, vegetables or frozen food like bloodworms, krill or brine shrimp. The minute he was fed any pellets you could bet he would be upside down within the hour. Sometimes this is the case, and you just have to adjust what you feed them.
Hope that helps!