Please help -sick tetras?

[Gio_St]

2 weeks ago I introduced to my tank a couple of flame tetras. All seemed healthy when I bought them but now one has a swollen stomach while another one has a slightly curved spine, as per the photos below.
The one with the swollen stomach is like this for 4 days or so and just noticed the deformity on the other one this morning.
Yesterday I went back to the store I bought them from and noticed quite a few tetras with a curved spine (and this led me notice the same for one of my own tetras).
After having done some digging online, seems their deformity is caused by NTD and, if so, potentially they were carriers when I bought them (albeit at the time there were no symptoms)?
Other species in my tank are zebra danios, lambchop rasboras, otos, and amanos. Are they all susceptible to NTD too?

Many thanks,
Giorgos

 

[PARAGUAY]

Definetly dont hesitate return them to the store.

 

[Gio_St]

after feeding my fish with daphnia, the one with the swollen stomach now looks much more natural (in terms of shape that is), so much so that I can hardly distinguish it from the rest of my flame tetras. I am aware daphnia can help with constipation but I feel the fish was too swollen for the deformity to be due to constipation? I was worried there was fluid accumulation..