Hi,
Could you please provide more details?
1. Water parameters and temperature
2. When was the last water change conducted? How much water was changed?
3. Test kit used
4. Foods and frequency of feeding
5. When was the last fish introduced? Was it quarantined?
6. Point a flashlight. Do they exhibit gold coating?
7. Any health history record?
8. Have they spawned before?
9. What have you done in this situation so far?
Here are a few things to consider.
1. Stress. When you attempt to do a large water change (in an old tank syndrome scenario), the fish quickly becomes stressed especially if your hardness levels and other parameters suddenly fluctuate. The result would be frayed fins, listlessness, white spots seemingly sprinkled like flour on the fish and gasping. Try to read on velvet disease in one of the sticky threads here entitled, "Freshwater Disease Guide".
2. Bacterial infections. I can see their skin seemingly peeled raw.
3. Anorexia. It looks like your fish are very thin. How did they get skinny like that? If you observed white stringy poop before, then your fish may likely harbor internal parasites. This will complicate matters already if we're already dealing with possible fluke infestation (quite prevalent among goldfish) and bacterial infections.
