My GSP suddenly died

[PettitM90]

Hi I'm new to this whole platform and keeping puffers but we are currently cycling a 55 gallon for a new home for fish

While doing so we came across a crazy deal at magoos for 2 baby GSPs and their whole (temporary) setup for super cheap (which makes me fear they knew something was wrong) but we brought them home yesterday our 55 gallon isn't quite where it needs to be so yes they are sadly currently in a 5 gallon that we bought them in all the levels are good

Ph is a little high about 8.6
Ammonia 0
Nitrite 0
Nitrates 0
Salinity last checked by magoos was 0.004

Tank size sadly currently was the 5 gallon and inhabitants was only the 2 baby Gsps

The tank was just cleaned by magoos before we bought them but I've tested everything myself except salinity we just got them Sunday so 2 days ago

But yesterday they seemed happy and fine they're colors were good they both ate bloodworms and were seemingly swimming fine then today I woke up and the one seemed like it was kinda coughing the other one was resting. But I read they can cough for a little and be fine so I laid back down as I work 3rd shift tonight well then my husband woke me up and said the one was gone. I just wanna understand what could have happened. If it was something I did or didn't do. I know their tank is currently too small but they seemed happy for the most part their colors were good and everything up until today. It was all so sudden and my heart is sad.

[Pufferpunk]

Never purchase a fish for a tank that is not cycled! Also, figure 10g for each 1" of this fish species.
library/water-filtration/emergencycycle/
library/water-filtration/fishless-cycling/
library/puffers-in-focus/an-introductio ... d-puffers/