Rainbow Shark - New Tank - Other Fish Keep Dying

I switched my tank from 30 to 40 gallons. In the previous tank, everyone lived happily. My friend who switched it did a 75% water switch, and at first, I thought this was the problem. The tank has now cycled for over a month. One by one, the fish keep dying—I mean literally one by one. Each morning, just ONE new fish is dead.

Desperate for answers, we went to the fish store, and they said the cause was unknown. They tested the tank water, and it was perfect, which was a relief but also left us with no clear direction on how to solve the issue.

My friend who takes care of the tank says it is because Ruby the Albino Rainbow Shark must be the cause. Ruby is about three years old.

Ruby has had a change of behaviour. Specifically hiding more in the top of the root system in the photo on the right side.

The first fish that died were fish from the previous tank. Tetra's, Upside Down catfish, Snail

The new tanks tetras all died - 3 of them

The Platys are being killed one by one-there are 2 of 5 left

The catfish is dead.

The Kohli Loch and Molly's have remained alive through the whole process.

The 6 Dano's are fine.

Any ideas?