Spotted Puffers - Sick

Question
I am new at aquariums.  I have a 12 Gallon Marineland Whisper with the filter at the top of the tank.  I set up the tank according to the directions and have two 1 1/2 inch spotted puffers.  The water tests perfect including the amonia. I ran the tank for three weeks before adding fish.  One puffer has bitten off the fin on the side of the other puffer.  Now neither one of them is eating anything including high quality flake food, Krill or Baby Shrimp (canned).  Last night I performed a 40% water change - again everything tested perfect. I also replaced the filter. I have only plastic plants.  I have this set up as a brackish tank. My two questions are (1) Is there anyway to heal the place where the fin is gone?  This puffer swims close to the top of the tank although after the water change he was moving around more this morning and (2) How can I get them to eat?  As they grow larger, I had planned to move them to a 37 gallon Marineland tank with a Penguin filter.

Even though these are small inexpensive fish, I do not want them to die because of my poor care.

Thank you for your help.  By the way, it seems almost all the other expert volunteers are on vacation at the moment.

Thanks again,

Margo

Answer
Hello Margo,

Sorry for the delay, like you said, seems like a lot of folks are on vacation or are 'retiring'.

The tank you ahve now is way too small for those fish.  (even one fish, that tank is too small)

I would not try to keep them both in that or even the 37 gallon tank.  They will compete for food, even if they dont really need to compete, they will instinctively compete.

AS for treating them, I would just try to feed them healthy / meaty foods.

Good luck