Back From The Dead.

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.
 

Great.

I was just going to suggest that another alternative would be to try a treatment for bacterial infection - but one thing I did notice when having a look at the Interpet swimbladder and anti-bacterial infection treatments a few weeks ago was that they have exactly the same ingredients as one another but the swimbladder one is at double the dosage, so you've definitely made the better choice there.  Fingers crossed that this works for you / your fish.  Keep us posted!  If need be, we can advise on alternative treatments if that doesn't work - but I really hope it does.

Coincidentally again to you, it was my healthiest fish that was suffering.  Indeed, you can never tell.

Do you think it’s worth an update on the medinas sticky thread re this @fcmf?

I would, if someone could confirm for me the ingredients and amount of each?
According to another forum thread I found, Interpet Anti-Internal Bacteria contains Bronopol 523mg, Formaldehyde 1000mg, Benzalkonium Chloride 250mg.
Perhaps @notungz can confirm that the Interpet Swimbladder treatment contains identical ingredients but at Bronopol 1046mg, Formaldehyde 2,000mg and Benzalkonium Chloride 500mg? (I'm now beginning to doubt myself and wondering if it was only the B.C. that was at twice the strength...)

I'm afraid to say that my Cherry Barb didn't make it. He put up quite an effort and continued to fool me into thinking he had died only to come back to life again on several more occasions.

Re: fcmf about the ingredients in these treatments. I can confirm the figures that you give for the Anti Internal Bacteria treatment are correct (per 100 mls). I think my Swimbladder treatment might be a bit old because I can't find any figures on it.

I think that I might treat myself to some more fish to cheer myself up.

Thank you for the advice.