I am very worried about one of my female pea puffers, her stomach looks like it has caved in on itself.

They only eat snails and defrosted bloodworms. I make sure to keep a steady supply of pond snails in the tank and supplement that with bloodworms twice a week. Two weeks ago things were hectic, we were gone a lot and had a bunch of family stuff going on so none of the fish got fed anything aside from flakes or pellet foods. Everyone else survived without issue except this one puffer.

I have been feeding that tank bloodworms every night since. She is less listless than she was earlier in the week but she hasn't put on any girth yet and her stomach is still caved in. I have been watching her eat, she isn't as aggressive an eater as she used to be - she seems unable to eat a snail like she used to.

Should I treat the water for anything, maybe internal parasites, or do you think she just didn't get enough food? I'd hate to lose this little girl - they are so much fun to watch. And QT isn't really an option because I have my swordtail fry and endlers in the QT tank. Even though she doesn't seem to have enough energy to attack a snail I wouldn't feel right risking the life of the other fish.

Jen, if you need a QT tank, I have a 10 with a filter you can borrow.

Or if you've got one, I'm currently using a 5-gallon home depot bucket as a QT/Hospital tank for one of my sick fish

Try BBS. I was talking with somebody at the picnic in Columbia and he said BBS is the best food for them. Golden pearls imitate BBS. You could try that, too. Or give them a big earthworm.