I agree with the above. It isn't a sure thing that it was an ammonia spike. It could have been stress, which has many different factors. If you copy and fill out the form here
so we could have a bit more information, it might be a little easier for us to ponder the different things that may have gone wrong, but don't be too hard on yourself as you did what you could.
In any case, you may never know what happened. Some fish just pass away mysteriously for no apparent reasons, regardless of the care provided, but I do agree that it helps to maybe try to figure out some things that you could have done differently to ensure that it doesn't happen again.
I am very sorry for your loss. It's always hard to say goodbye and to not blame ourselves. :(