Is the mass on both sides or just the one? To me, it looks like a growth or similar.
Laying on the side usually isn’t a good sign.
kno4te
If you find your fish at this stage, sorry to say it is too late. She is going to die. Mine died. I had a lot of trauma wiht ny puffer fishes. It is not easy to keep this type of puffers. I managed also to breed them, but could no go beyond 45 days. Specially the females are too difficult to keep, this is because when they eat food, they produce eggs in their bellies and a male puffer is needed to empty those bellies from the generated eggs and the male puffer cannot do that without properly functioning teeth. If the teeth are overgrown then it cannot hang to the belly of female with his teeth, so the female puffer eventually bloats and dies. The male puffer however if the teeth is overgrown then forget about being able to fix it, becuase the mouth structure of these puffers are so complex that their teeth are impossible to trim properly, so the only solution is not to let the teeth overgrown and that can only be achieved by properly maintaining the supply of posthorn snails as at least 50% of their diet, otherwise overgrown teeth are inevitable. I gave up keeping female puffers so I only have the single male puffers and I will keep him as long as he lives but never again would I choose this type of puffers.
