A quarentine tank is good. Please have a mature filter running on it. The bacteria in mature filter media will keep the ammonia and nitrite levels under control...VERY important! If you started the tank/filter up with the fish in it and all was new (meaning no mature media) you will certainly have had an ammonia then a nitrite spike. Both are very poisenous to fish and very likely contributed to the problem. The filter on the original tank should be cycled by now if you have just rinsed the sponges, etc. in used tank water. Maybe, if need be, use part of these sponges in the Q-tank fiter to seed it w/ the good bacteria. Test your water for ammonia and nitrate levels. If they are zero continue with declorinated water changes at least every week in a healthy tank, as much as once a day if levels are detectable.
What are the sizes of your tanks and how many fish in each one? You don't want to be overstocked.
In a different vein of thought, your Moor could be constipated by eating to much dry food which expands in the stomach. If he's eating try feeding only peas for a few days. Cooked them just slightly in the microwave, cool, remove the outer hull.
Some diseases can cause this condition (not an area of expertise for me). Google goldfish diseases and do some research, remembering that it's not an exact science. Prevention is easier than curing.
Good luck to you and your Moor and feel free to ask further questions. This forum isn't frequented often so you might get more help in the general freshwater or newbie forum.