Sorry to hear that your little friend died. You did all that you reasonably could to give him a good life.

Just in case anyone needs to know a compassionate way to put down a fish...

Scoop him up in some of his own water, with as little disturbance as possible, then put him and the water into a bowl or ziplock bag. Insert into freezer.

Their nervous system quietly shuts down without panic, and it is simply over by the next day.