Figure 8 puffer died mysteriously, can someone help me understand?

[Puffer8noobie]

Hello Everyone,

Can I get some opinions on why my figure 8 puffer died? I bought him from the local fish store maybe 3 weeks ago and he was pretty healthy.

Here's the info I believe could be useful:
- I was feeding him mysis shrimp almost every day, and here and there I would skip a day to give him a break
- He was eating fine and seemed energetic, was interacting well with the other fishes, the only fish that sometimes gave him a bit of a chase was my red tail but he hides most of the time and isn't really aggro, I noticed they mostly ignored each other
- He was also friendly with the shrimps and my tetras
- He was in brackish water at the store, but I don't have brackish water setup at my house, so I acclimated him over 6 hours with the drip method, he seemed ok from the very first day
- Water temp is around 24c-26c
- I do water changes once a week mostly
- All my other fish are healthy / normal habits

Then the issues started happening around 3 days ago:
- Loss of appetite
- Started being lethargic and I noticed him kinda hiding and hanging out in the bottom of the tank
- I tried feeding him a couple times but would completely ignore food
- I even went after getting some snails and he wasn't interested either

I attached some pictures so you guys can see it, when I fed him he would eat until his belly was round.

My question:
- Do you guys think his teeth was overgrown because of the lack of hard foods for the first 2 weeks? (I only got snails for him a couple days ago but he was already acting weird at that point)
- Was it most likely a parasite?
- Any other ideas?

I'm really bummed out because he was the star of the tank, I was so happy when I saw him acclimate nicely at first :/

Any ideas would be appreciated, I'd like to try again in the future but only if I can get to the bottom of this.

Thank you

[Pufferpunk]

Sorry it took me so long to respond I was having trouble with my account.
2 weeks isn't long enough for the teeth to grow long unless they came that way.
What tank mates are in there?

[Crococlaire]

I would say it could be parasitic. Or fungal. It’s hard to know without a necropsy.