1.- water parameters: ph: 7.7, NO2< 0.3 mg/l, NH3/NH40 mg/l, and 1.002.
2.- Tank size and a list of ALL inhabitants: 204 lt, 4 puffer (2 nigroviridis & 2 fluviatilis at 2.5") and 1 Scatophagus argus.
3.- water change schedule: he changes 15-20% every week and does a 30-40% once per month, he does nor use chemicals at all (his tap water does not contain chlorine or chloramines); He feeds them with zophoba Morio(live), snails (live), prawns(frozen), Tenebrio Molitor (live), mussel (frozen) every day uses a difrent food.
4.-What changes you've made in the tank in the last week or so: he add the Scatophagus argus a month ago, and he said that the puffer ate the mussel and after 2 days the protuberance appears.
5.-How long the aquarium has been set up, and how did you cycle it? Yes it was properly cicled, has 2 years working and the sick puffer has 1 year in the tank.
Pictures:
Here is how it started:

The became like this:

Now, the poor guy is like this:

¿what could it be? i suspect about a degenerative melanoma, like in angle fish, but dont know why it happens or how tu cure it.
Please Help!
¿rise salinity? ¿do 30% wc every week?