There may have been something in the air/water,
@notungz - I had a cardinal tetra with a similar experience at the same time! Full saga here:
https://forums.thinkfish.co.uk/fish-health/cardinal-tetra/20/ The one piece of advice I would give is of the benefits of aquarium salt / sodium chloride for helping with buoyancy and osmoregulation. I had used this to good effect with goldfish in the past but had never used it with tropicals as I assumed they were too sensitive.
A fish vet actually recommended it for this particular cardinal tetra's situation, and I was very apprehensive, so added it ever so carefully - literally one crystal every 15 or 30 minutes. The fish vet advised 6mg/litre but, doing some googling as I always err on the side of caution, I read that sensitive species might be better with 0.5-1.5 mg/l (PFK article). As it happened, I measured out 0.5 mg/l but actually only used 0.25 mg/l as the problem rectified itself by the time I'd put that much in. Check Reply #21 in the above link for before and after photos.
Some people recommend Epsom salts (magnesium sulphate - which I've used twice with adverse effects), some experts and vets recommend aquarium salt (sodium chloride). There was a small discussion about this varied advice elsewhere, and it seems that folk sway towards sodium chloride as being more beneficial.
Depending on what swimbladder treatment you're using, you may not need sodium chloride as well, as it may already contain that. Check here for the brand you're using:
https://forums.thinkfish.co.uk/staff-room/fish-meds-ingredients/msg50195/#msg50195 However, if that treatment doesn't work, I'd definitely recommend giving the sodium chloride a go.