No forgiveness needed. You learned something today... consider your penance paid in full

I trust you removed the injured fish to avoid any harmful effects... though a tasty snack for other tank inhabitants, but a newer established tank may not be able to deal with the waste load of a decaying fish efficiently enough, best to remove him and avoid the inevitable, or if you have another tank set up to serve as a hospital tank if there is a chance to rescue him relocate him and keep your fingers crossed.

If it's too late we're sorry for your loss, not much more advice I can think of to pass along.