Saddleback Puffer Dead Confused



Hello first post on the forum and its not a pleasant one. It's nice to meet you all!

So for reference the tank is a 45 bow front, been set up since December and all testing shows levels to be normal, no ammonia, nitrite etc. Salt is within normal ranges. The only thing off a bit was my ph, sitting at 7.8. This tank has a canister filter, heater, one power head. Livestock would only be my mated clowns, cleanup crew and live rock.

So for the question...
Recently as of four days ago I purchased a small saddle back puffer from my local store. He was active and showed no out word signs of of illness. I brought him home acclamated him and released him into the tank.
At first he actively swam around but soon he chose to hid by a rock. He would come out every day but hide most of it. He didn't eat but I've herd that often don't eat at first when newly in the tank.
Last night he took a turn for the worse and was breathing heavy and sitting on the sand. Today I've found him dead.

I'm wondering what went wrong and how I can learn from this to move forward. I'm not sure what happened to him. He never showed illness, didn't puff up, and looked fine outside of hiding. I'll be honest that this was my first time getting a puffer and I was very excited about it. Now I'm feeling defeated.
Do you have an advice? What could have went wrong?