Whats wrong with my black moor goldfish?

Those are cold water fish. Edit for additional I asked the cycling question because so many people asking questions are under the impression that cycling is setting the tank up and running it empty for a week and because you only had the fish 2 weeks. I didn't mean to sound insulting.

Obviously the fish was stressed about something and water conditions probably account for 90% of fish problems with symptoms you gave, so I had to ask. If the fantail wasn't bullying it and there were no other obvious signs of illness then it could have just been stress from the move. Fish get manhandled quite a lot before they make it to the tank.

A 20 gallon tank is a small tank for two fish depending on size. Black moors change colors due to age, as well as due to stress. You may want to get a water testing kit to check nitrate levels, etc and not just use cloudiness as an indicator.

The tank needs time to create a healthy bacteria cycle and the use of over the tank filters can cause issues if you change your filter regularly. (You throw away all that bacteria when you throw the filter away.) I would recommend an under gravel filter system if you don't already have one. Test the water to ensure proper levels, use an under gravel filter system, and remember to not over populate or to over feed a new tank.

It's possible you simply got two unwell fish from a chain store that didn't watch for signs of sickness or decreasing health.

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