All fish dieing

[Accordexi]

I do not blame anyone/especially the store that gracefully kept the fish for me while I moved

so here is my sad story.

My fish
5 tangs
1 emperor angle
1 coral beauty
1 clown
And a few little damsels

I dropped off the fish on Friday afternoon. All healthy eating like crazy and no signs of anything wrong.
The store acclimated them super professional. And put them into the holding tank. One of the tanks that has about 6 total tanks on the same system .

Come one week this Friday.
I went to pickup the fish when I was informed that all fish died. I took it as a joke and said how many did they sell...

To what they responded that it was for real.
I headed to the same tank that had my fish one week before to find it completely empty!. I then looked at the other tanks and noticed that other fish were there. All seem to be fine. They did show me all the fish by taking them out of a freezer.

Now here is the question. What would wipe out only one tank while the others were fine ?

The fish apparently died from Sunday night to Monday morning. I'm a little disappointed that I wasn't made aware of this on Monday.

I need some help to understand what could have caused this disaster?

All the fish that died had been with me for well over 2 years.

Anyone had something like this happen?

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