The swelling could be caused by:
1. Lymphocystis,
2. Damaged bladder or intestines, or
3. Trapped air.
If the puffer is having trouble balancing that would make internal damage or trapped air more likely (though these could be combined with lymphocystis).
3. Trapped air is the least likely cause, but this can be quickly tested and fixed by briefly (and gently) removing the puffer from water, as follows:
-- Prepare a large clean piece of cloth, soak it well in the tank water, fold it and put it in a clean plastic bowl (the larger the better).
-- Fill a large cup or container with tank water and set it aside.
-- Put clean gloves on your hands and rinse them well in tank water.
-- Use another large cup or container to gently scoop the puffer out of the tank.
-- Now, transfer the puffer to the bowl with the wet cloth.
If the bowl is large you can gently pour the puffer into the bowl. Otherwise, transfer him by hand, by gently placing your hand under the belly and lifting him.
-- If there was air trapped inside the body, it should be expelled within few seconds.
-- After 20 seconds or, if the puffer starts puffing then immediately, pour all the tank water you have left into the bowl with the puffer, take the bowl to the tank and pour the puffer into the tank.
1. Lymphocystis is a viral disease which cannot be cured but with good care it usually disappears on its own within a few weeks.
2. Damage to bladder or intestines would require a visit to a vet.
Good luck to you and your puffer.
LeoR
diodon.org