Usually if fish get a swim bladder problem they die.

It's usually over feeding by you, your mum/dad, brother/sister, or nephew/ niece that causes the problem.

I've never successfully saved a fish which was doing the death roll... until recently.

Start by cleaning out the tank and checking the pH is correct (the latter seems to be the main problem)...
just add bicarbonate of soda until your pH is around 7 (goldfish like that).

Add one drop of ClO2 (Chlorine dioxide) in water to the tank.

It's a small dose... but is sufficient to kill a parasite... or oxidise a toxin.

My fish stopped doing the death roll in less than an hour.

My tank is about 30 L... but chlorine dioxide has amazing potency even at very low levels.
It has about a 12 hour persistence which means it will kill most bacteria... as they lack the ability to resist oxidation... it's the dose that makes the poison (Paracelsus)

Animals and fish both have the enzyme catalase which breaks down oxidation products... so as long as you don't overdose they'll be OK.

It's now a year or two after this incident and my fish is still here. Admittedly he's just a lowly goldfish... which are the fairly hardy compared to many tropical fish.

Chlorine dioxide was created by using sodium chlorite (Na+ClO2-) at 26% and activated with one drop of citric acid at 20% (bought it in power form at the shop it's in the cooking section and mixed it 20% powder to 80% water)... or use 5 drops of lemon juice?

You've got very little to lose trying this as your fish will almost certainly die if you do nothing... and commercial solutions have never worked in the past.

Sample size N=2 (which is a bit small).