Hi Benlinda,
I don't know how sensitive your daughter is, but we had close to the same thing happen and my 5 yr old daughter is EXTREAMLY sensitive and when we lost our first fish in the new aquarium, she was heartbroken - almost as much as I was!!! Tears and everything. In our case she wouldn't have noticed a difference because we had about 10 fish, but she was the one who spotted him floating. What a better time than now to briefly explain death to a child. I would rather be the one to tell my daughter before television or other kids did!!! I personally thought the best thing to do is let her be there when we "sent the fish to fishy heaven". I explained to her the fishy was sick and God decided he would be better in Fishy Heaven. We even said "goodbye" and she wanted to be the one to "flush". She even said what a good fish he had been. - you know, the whole stereotypical "flushing a fish" drama thing - like I said she's very dramatic and sensative and she understood that it was okay to be sad, but that fishy was happy swimming with other fishes in fishy heaven, and was okay with it. -She did ask one thing though, and that was if the fish was where our potty was, lol, I did fib a little and told her there's a special pipe that takes the fishies to heaven!! Got to be creative! Trust your instincts - you know your daughter best! Good luck!
A.