You probably already did something because it has been so long. But in case you still want to know, here is my experience with pea puffers.
I have had a pea puffer for over a year. I have one female in a 5 gallon planted aquarium.
When I first got her she was very shy and was not used to people and didn't associate people with food.
She had to have live food in order to eat.
I fed her live black worms and either ramshorn snails or pond snails which I had a lot of in my 10 gallon. I luckily had been breeding rabbit snails and since I had been feeding them well, my other snail populations also exploded before I got her. Even then she still ate them faster than they reproduced in the end and I had dozens extra. If yours has never eaten a snail it takes them a few times to figure it out. They suck the snail right out of the shell.
It took months for her to figure out that frozen blood worms were also food. Once she did she would come right to me and eat one or two. If the blood worms were really tiny she could eat more, but usually not. If one was big then one was all she could eat. You have to feed them every day because they are so small and you don't want to over feed them. You want their tummy to be rounded but not huge.
You could have one or three in a 12 gallon. Or that is what my LFS says - they say two will fight and you will end up with one. They say a trio will do okay. They need lots of hiding places and the line of sight broken up. I have only had one though because it is easy to be stocked just right and not worry about water quality.
They are really fun to watch hunting in the plants and gravel looking for snails or worms to eat. Once you are feeding blood worms though they change and when they see you they start begging for food. Of course when they don't see you they are still hunting even though nothing is in there
If you have lots of plants you can watch for a while before they see you.