Need help keeping new fish alive.

[GreenSpottedPuffer]

Can we get a bit more info on the system?

How much LR? Skimmer? Water parameters?

Is there or has been ich in the tank?

As much of the history of the tank that you can honestly think of. Something may come up...otherwise it could be a million different things.

It can be so hard to figure these kinds of sudden deaths out without more info.