Thin fish, not eating

[Karen B.]

I ordered WCMM from my LFS (no breeder in the area and we are in lockdown so couldn’t go pick them up).

One of them is really thin. Its belly is just flat, not rounded (but not caved in either). He mostly swim by itself toward the surface.

Yesterday it was unable to eat my Hikari fancy guppy pellets. Today I gave them frozen bloodworms that I thawed in Seachem garlic guard and the little fish would take one, munch on it for 10 seconds and spit it. Same with a smaller piece.
I isolated him with a fishnet and tried again the fancy guppy hikari food but it was too stressed to eat.
I live in Canada, we do not have access to any fish medication so I was afraid with all this stress he would develop something like ich. I isolated him in a small pitcher with aquarium water, a plastic plant from the tank and a air stone. I have tried giving him food again but it mostly stays at the bottom of the pitcher.
I am at loss for the next step? Should I put him back in the tank?

I have frozen daphnia, should I try these before? 
 

Thank you everyone. 

[Travis]

If there are no external signs of disease and the rest of the tank is doing well it could be an internal parasite causing it to waste away. Without access to proper aquarium meds I am unsure how to treat this. If possible I would keep it isolated from the main tank until you see more change.

[Daniel]

Once a fish has a 'caved-in' stomach like that, it is rare for them to make a come back. I don't know any treatment unfortunately.

[FriendlyLoach]

I would be careful, I have herd that fish TB can do that, so I would avoid putting your had in the tank and definitely if you have a cut. 

[Brandy]

I just want you to see this thread. Sometimes, there really is nothing you can do.

https://forum.aquariumcoop.com/topic/342-tetras-gaspinghelp/?tab=comments#comment-2037

As an update, I have a thriving school of 20 rummynose tetras, and no further losses. And I hear Cory's amazing school is doing well as well.

Most fish are either farmed or wild caught. Both situations mean they can arrive with disease or parasites. We help as much as we can, but there is truth to the fact that no one can save every fish.

[FrostiesFishes]

I would try Fritz Expel-P. It’s for internal parasites, worms and wasting disease. Put them in a qt tank.  It’s basically just Levamisole. But probably easier to find then pure Levamisole.