Something very wrong with my tank!

Hi everyone!
I got some new fish about a week ago. It is a 20 gallon tank with zero ammonia, zero nitrites, zero nitrates, and 7.5 ph. I should have quarantined the fish, I will do that in the future. In the past week, six fish have died. Three harlequin rasboras, two Cory cats, and a betta. None of them show any sign of sickness. They just die. They are perfectly healthy until I find them on the bottom of the tank and still they look fine, except for, you know, being dead. Please help!

Now I only have three harlequin rasboras left and one keeps attacking the other two.

Happy day..I am sorry for your loss and hope that a happy memory can lessen the grief you feel this day. By chance did you do a water change and not dechlorinate?

Pictures of the fish?

What is the pH of the water at the shop?

You might have heavy metals or chemicals in the water that is killing the fish. Or the fish might have just come into the shop shortly before you bought them.

Do you have any cream, soapy, hand sanitiser, oil, grease, perfume or anything else on your hands?

Do you use air fresheners, hair spray, deodorant, smoke, paint or anything else in the room with the fish tank?

Do you use a bucket specifically for the fish or do you use any bucket from around the house?
Does anyone use the fish bucket for anything besides the fish?

Add some carbon to the filter for a month and see if it helps.

Picture of the whole tank please.

@Colin_T
I don’t have pics of the dead ones but here are the live ones: hard to get a pic when they swim around so much.
I don’t know the ph of the shop water.
No, I don’t.
No, I don’t.
I use a specific bucket only used for fish.
And I’ll put some carbon in there.

@itiwhetu