wrote:

Hi,

Guess what???

I got to work this afternoon and the three fish have been moved into
the tank...

Last night before I left I tested the water for amonia and got a
reading of 2.4, so did half tank change water.

That still leaves 1.2ppm (I assume you meassured in ppm) of ammonia in the tank.
If you're fish are frail that can be lethal!