I am so sorry but the water is "very good" will not help me much. I need numbers from the tests.
ammonia should always be 0 no amount of ammonia is acceptable to your fish. Nitrites should also be 0 as this is the next step of from ammonia and is toxic to fish. Nitrates should be kept below 20 at all times but sometimes this is hard to do in some areas of the country where the drinking and tap water straight from the tap has more than that in it. In those cases the only things I know that will bring that down in a hurry is boughten store Spring water or Live Plants in your tank (they use the nitrates for fertilizer and will bring down the level but takes a while).
If you took the fish from the tank he will need to have a heater in the tank you put him in and since I am doubting that it is filtered the water will need a 50% change daily. You can treat him with Maracyn2 for it but put a whole packet in the water each day when you have changed the water.
Dropsy is not actually a disease but a symptom of a problem and that problem is kidney failure in your poor little guy. I will say that there have been some fish who have come through it but there is a high rate of fish not coming through it so it will need to be treated quickly. He will need continued clean water, warm conditions and you can try the medication and you can soak his food in garlic juice to help boost his immune system. This would not be a bad idea for the rest of the fish either. You can get minced garlic in its own juice at most dollar stores and at the grocery store or anywhere they sell groceries. Ask if you do not see it. It will be kept in glass jars, not plastic and will look like chopped grains of rice in liquid. Garlic is good for your fish and will help them stay healthier.
I do hope that your little one will be okay and we do want you to keep us informed of how he is doing. The clean water and good conditions will help but the med will be a help too. I shall send you good hopes and welcome to our site.
By the way, the fish in your tank may not be too compatible. Is there any chance that there is fighting going on in there and the little guy may have been injured?
Rose