lost my red devil

Evening guys.

Im sorry to have to say my small red devil died today after only having him a week.
Cant understand what happened as for the first few days he was very active, eating well and mixing with my other angels.
2 days ago i noticed he became withdrawn and stopped eating.
Yesterday he was just sat at the bottom of the tank and i began to fear the worst.
Today i came home from work and found him dead.
All my other fish are very very healthy and ive checked my water and its spot on.
Why he died i have no idea.
Anyway ive been to my local fish store and bought myself a small koi and a marbled to chear myself up.

Sorry to hear of your loss.

I hope you've put the new ones into a quarantine tank!

It's weird that, when everything seems fine but the death lets you know it wasn't. I go as far as ph testing the water in the bag which the fish are in too. Sorry for your loss, hopefully your new fish will be stronger.