55 Gallon Tank Skinny pea puffer

So I have 8 pea puffers in my 55 gallon, which will get more plants soon, and most of the pea puffers are doing amazingly well. There’s a few fair sized ones and the rest are pretty small but there’s one in particular that’s worrying me. I got 2 at first because that’s all my lfs had and 6 more later on. I know as a fact the really skinny one is from that group of 6. It’s not bullied, there’s actually 0 aggression in the tank like rarely even a chase, and I offer them frozen blood worms, frozen brine shrimp and live pest snails twice a day but it barely accepts those and instead likes to pick on the biofilm on my driftwood and I’ve always read that they’re pure carnivores. I kinda doubt it’s a parasite since the store I bought them from is one of the best I’ve ever found (always quarantine and medicate the fish in a back room before putting into the sell tanks and a few fish have been there for like 2 months so they obviously take care of the fish) but it’s always a possibility. The most likely thing I can think of is that it’s because the 15 gallon it was in before I moved it to the 55 got a little nasty and was growing brown and blue bacteria/algae and that puffer is still recovering. I know exactly why that tank was nasty btw so they’ll be fine in the 55 gallon. Either way any tips on helping it recover?

 

In the wild these guys are susceptible to a parasite that shows the symptoms you describe and sometimes bring them in with them.
Although it's nice to here people say they have confidence in their lfs it's worth asking them if its possible that one may have a parasite based on what you say, its possible they treated for them but it needs doing again,
Looking closer at the picture he dosnt look as if he's wasting away, maybe he's just small, but still worth a chat to your lfs

 

If that’s the case I can setup my 15 gallon to treat it. Only thing I’m worried about is that I’m going on vacation soon and I’ll gone for a week. My brother will be home but he knows nothing about aquariums so my plan was to do a water change the day before I leave and just have him feed my 2 tanks. Only have three questions. Can the parasite spread or stay in my tank after I treat the individual? How would I treat it? And how long would the treatments take?

 

the stuff i use for puffers takes 10 days and they needs to be monitered
It's also no good for any type of invertebrate

 

Then I’m gonna have to hope he makes it through that week at the beach. If he does I can setup the 15 gallon aquarium to treat it and reintroduce him.