Need "HELP" My fish is sick

[SMOOOZZZ]

So we decided to buy a small tank 10 gallons and bought some freshwater Puffer fish. So far we have three, and one of them had a crooked front tooth and this morning I noticed it was no longer there. He ate blood warms this morning with no problem but he's not wanting to swim
around. He staying in the bottom, he's a darker green shade right now. Can anyone help?

 

[Try-na]

I don't know very much about puffers except they are aggressive & multiples shouldn't be housed in a 10 gallon tank. Also you just got the tank? Did you cycle prior to adding the fish?