These are always the worst losses when we know we could have done something.
I still think about the time I did a water change on a fancy goldfish tank and I did not use a strainer on the w/c pipe and one of the fish got sucked up against the tube, it took a while to die and I know I could have prevented the mistake.
Learn from the mistake and ensure no others are harmed, it is the best you can do.
Sorry for the loss.
