I own a greenspotted puffer and the belly has blacken what the problem here?

Answer 1

It could be serious or it could be nothing at all and there are simply too many variables to tell which without much, much more info from you. I would like to suggest a link:
http://www.wetwebmedia.com/BrackishSubWebIndex/gspdisfaqs.htm

The above page have several FAQ's dealing with black on or around the belly on a green spotted puffer. No doubt as you read through it and some of the links they reference you will be able to find out not only the answer to your question, but loads more info about your cool fish.

Hope this helps

MM

Answer 2

Check your water parameters (ammonia, nitrite and nitrate) and make sure the levels are correct. Are you keeping him in brackish water (a step between fresh and saltwater?). Many of the male puffers get blackening around their bellies when they're stressed. Water conditions are usually a major cause of stress in fish. Other times though, with the puffers, it can come AFTER a water change or even if they're anticipating being fed. Sometimes it's not stress stress..but anticipation stress. As long as he's swimming and eating fine, it shouldn't be a problem.

Answer 3

Sounds like Ammonia burn. Do a emergency 30 percent water change and check the quality of your water (ammonia, nitrite, and nitrates) If the fish is ammonia burned it will probably live unless its an extremely severe and prolonged case.