F8 is always skinny

[abcdefghi]

I have had a Figure 8 puffer for about 6 months now, he (or she) is typically very active, a voracious eater, but no matter how much it eats it goes back to looking really skinny. I feed him snails, shrimp pellets, frozen blood worms, frozen brine shrimp and am working on breeding ghost shrimp to feed as well. If I drop food in for him, he chases after it, eats it and gets a nice round belly. But, within a few hours he is back to looking sickly thin again.

This morning I came downstairs and honestly thought he had died. He was sitting on the substrate in the back corner of the tank, looking terribly skinny, I got ready to tell my son the bad news (it is his fish, he LOVES it). Then I notice him moving, sure enough, he is up swimming around. I dropped in a shrimp pellet and he went right after it and ate it. He is as curious as I would expect him to be, for example I put in a different thermometer to double check his water temperature, and he is swimming around it, looking at it and trying to figure out what it is (likely figure out if he can eat it).

But, I need to figure out why he gets so skinny. I am not sure if it is parasites of some kind, and if it is, I am not sure how long he could have realistically lived with them. I did treat for parasites when I first picked him up from the store, but maybe the treatment did not take fully. Any suggestions or help would be much appreciated.

Thanks.

[Pufferpunk]

library/hospital/internal-parasites-pre ... treatment/

[abcdefghi]

Sadly he passed away overnight.... Since I did initially treat him for parasites when we first got it, I can only assume that the initial treatment did not clear out all of the parasites, ultimately leading to his demise? He went downhill incredibly rapidly from Wednesday to Thursday, and even though he was still eating, I suspect was too far gone at that point.

[Pufferpunk]

How many days did you treat? Did you medicate his food?