I'm really sorry to hear what happened. =(
When I was younger, I always wanted a pet. In particular, a puppy. I didn't get one until I had finished school but she was my baby and I loved her, she went everywhere with me for the first few months!
To cut a long story short, I went on holidays and left her with a friend I had known just that year from uni. She escaped the yard, was hit by a car on a busy main road and didn't survive the night.
Its terrible hearing bad news like that but I have never felt any resentment or anger or anything at all negative towards my friend. It was not her fault, and I would never hold her at all responsible, although I think she felt really guilty about it.
Basically I guess my point is, just be honest and sincere. Tell her what happened. Tell her you are sorry about her fish. And let time do the rest.
Lol in between feeling really sad about my dog I spent a lot of time worrying that my friend was worried that I was mad or upset with her. So please don't worry or blame yourself. It could have happened to anyone, and may have been, like you suggest, completely unrelated to anything you did directly.
Good luck with telling your friend.