Tomato clown

I have a juvenile tomato clown I transferred to a 40 gallon breeder 2 days ago that I’ve cycled for a month, perimeter in the tank are perfect, and he was the only one in there. This morning I noticed he was in an upright position at The bottom Of the tank, I scooped him out and put him back in my 150 gallon tank( perimeters also good) when I put him in there he sank to the bottom gasping now dead.... wtf?