My Goldfish seems really healthy but he/she has black stuff at the ends of long fins in the back. What is it?

All goldfish are actually born black. The black fades as the fish ages, some sooner than others. Does the "black stuff" look like it is attached to the fin? or is does it just look like the fin is colored black? If the fin is colored black, that is just the tail end of the juvenile coloring fading. I have a young fish that is just finishing the color change. She was solid black for just over a year, then started to orange. In two weeks, she went from solid black to solid orange with white fins that have an edging of black still. Also, if it is a "feeder" fish or a comet, you can usually tell if it is male or female from the shape of the fins. Females have shorter, rounder fins while the males have the longer flowing more forked fins.

It is possible that the marks are just natural coloration. but black marks on the fins could also be a sign of ammonia burns. Ammonia burns happen when the ammonia in your tank spikes to levels that are toxic for your fish.

The immediate solution is water changes (50% of the volume) every day for the next several days. That will remove the ammonia.

The likely causes for ammonia burns are overcrowding (the tank is too small for the number of fish in it) or poor water conditions or both. Goldfish need 2 gallons of water volume per inch of body length. A 2-inch fish would require 4 gallons of water. So A ten gallon tank could 2 2-inch fish comfortably.

If the tank size is ok, then the problem is the water. Is your tank fully cycled? If not, you will need to do daily water changes for a few weeks until the benign bacteria that make up your biological filter get established. Once that happens, the ammonia spikes should stop. The website below has more information on that.

Even with a fully cycled tank you should be doing water changes (25% to 50% of the total water volume) at least a few times a month.

Keep in mind that the black marks are scars...so clearing out the ammonia wont make them go away immediately. But they will fade over time and eventually disappear. I had an Oranda with ammonia burns once, and they cleared up within a few months.

I really doubt it is any danger.

When u bought it was it already like that?

It should be fine if anything happen just add some sea salt to your fish tank every time you clean it.

I am sure it your goldfish genes

it might be how it is, like if your fish is a calico they're supposed to be multicolored, but if it looks really abnormal then it could be a disease =P check the pH, ammonia, nitrites and nitrate levels to see if there's anything wrong with the water

a picture would be helpful