We've recently brought two tiny gold fantails, they are really lively and fins up etc and eating OK but suddenly over past 4 days their tails and fins are turning black, and they have odd spots round the eyes 9 not in eyes they look ok) and gills - we ran cycle for ages etc, and test water doing water changes every couple of days as the ammonia always seems to be high no matter what we do - filter running OK , so confused as to why ammonia won't go down after running tank empty for three week before ! Are fish on way out? :( help !

Ammonia would be the cause of the black fins.

Take a water sample into your local pet store (aquarium specialist if you have one) and get a full water test done.
This will either be free or they may charge a small fee for the use of their testing equipment.

They will be able to reccomond the best product to add to your tank and the amount you will need to put in.

Gold fish will change color till they age a bit. I had a totally black one turn gold, you just never know what the end color will be. I also keep koi and they change as well.

If you have high ammonia levels add "ammo lock" to the water and "ammo chips" to the filter. The water will still test positive but it will be harmless. Stop all the water changes as this keeps slowing up the bacteria growth that consumes the nitrate levels. If the fish seem healthy then do not panic.