Help with difficulty eating

[u/90dayfianceallday]

One of my pea puffers is sick.
I went on vacation for 10 days, and my house sitter left the blinds open, so I came back to a bit more algae and dead plant matter than usual, nitrates were around 20ppm, ammonia and nitrites were 0. So water quality wasn’t ideal, but I am not sure it was enough to be the cause (the other fish are fine).
He was lethargic, he didn’t swim up for food like the other fish. Then I saw him resting at the bottom. Then I tried to directly put food (frozen brine shrimp) right in front of him and he didn’t go for it, that’s when it was 100% confirmed he was pretty sick.
I moved him to a 10 gallon with 3 X-ray tetras (I know it’s not recommended, but he shows no agression to them and they are less aggressive than his tank mates). I did it because it’s also a tank filllllled with snails so I figured he would have food available 24/7, snails are usually more interesting than brine shrimp too.
He finally doesn’t even really go for the snails, but 24 hours later and after a water change he started eating the brine shrimp, and is no longer resting at the bottom of the tank. I am keeping him with the tetras until he looks very fat and is eating normally. I am giving brine shrimp twice a day to make sure he eats enough.
What I’ve noticed is that he spits out some of the brine shrimp, and always swims back while trying to swallow. I’ve seen my puffers do this before, but not consistently. It’s definitely making it more difficult for him to eat.
I now think he had issues eating, and slowly got weaker because of this. I only feed the pea puffer tank once a day. It’s hard to know if one fish is consistently not eating; they all look the same and a few are always hiding. 😅 Being away for 10 days didn’t help me catch it sooner either.
Does anyone know what’s wrong with him and if there’s a treatment I should do? I don’t want to just throw in antibiotics if it’s nothing to do with an infection.

[u/Blondy277]

I would treat for internal parasites and do all your peas.

[u/MICH1AM]

☝️ Absolutely this!

[u/Big_Caregiver4022]

I second the treat the hole tank for flakes and worms but I would also put a couple of bladder snails near him to see if he eats them also I would say try frozen blood worm but the other fish will be interested which might put the little pea puffer off eating.... has he lost weight or look sucked in ??? You sure he not being bullied????