My aquarium fish has a swollen stomach!?

I have a cold water aquarium with 5 goldfish in. They bred but only three babies survived. They are now about 1 inch long (6 months old), but one is showing a very bloated stomach, no pine cone effect, but more like a bag of marbles, bloated but also localised bulging. All other fish appear healthy, the water is crystal clear, well ventilated and full of plants. There has been no introduction of external species. Any suggestions?