Sick Green spotted puffer

Question
Hi, my green spotted puffer is sick. His tail is curved and snug against his body and his belly is constantly black. He can't swim straight at all. I was wondering what's wrong with him and is there anything I can do for him? Thanks a bunch, I'm sure you can help, you've been great with my questions so far! I really want him to live hes my favorite fish!

Answer
Hi Will,
It could be many things. But most likely it is water quality related. Is there any way you could test his water for ammonia and nitrites and nitrates. Take note of the readings. These levels should always be as follows- Ammonia-0 Nitrite-0 Nitrates- 20 or less. A water change just as a precaution wouldn't hurt.

Also, is he a new addition? If so, he may have been stressed by a switch of water chemistry between the pet-store water and your tank water. Also Green spotted puffers are actually brackish water fish. They thrive best in water that has had some salt in it and as adults they do best in fully marine conditions.

Try adding a little bit of aquarium salt to his water after testing it and doing a water change.(Always pre-dissolve it in a cup of tank water before adding.) Make sure he has some places to hide too to make him feel more comfortable. Make sure his temperature is warm as well(at least 78-80F)

Better yet slowly convert him over to brackish water. For that you'll need a hydrometer and marine salt mix such as Instant ocean. You'll do small water changes with about 1 gallon of brackish water (make sure all salt is dissolved ahead of time) per 5 gallons of water taken out. I've used about 1 level tablespoon per gallon.
Gradually increasing this by the week until he is at the desired salinity range. (Green spotted puffers prefer - 1.005-08 for small puffers of 2 inches and below, Medium puffers from 2-4 inches about 1.010-15, and large adult puffers - 1.018-22)

Best wishes,
You can read more about green spotted puffers here-
http://www.aquariumlife.net/articles/tropical-fish/64.asp

I hope this helps!
Karen~