since you've already made her mad from one email address, take the kinder approach from another address.
say something like "i've been looking for just that kind of fish"
she'll assume you mean you were looking for that "rare kind of fish" but you actually mean you were looking for that "stuck in an unhealthy environment kind of fish"
dont tell her what you're planning on doing with it, just tell her you will give it a nice big happy home and you're so excited to see it.
and praise her for learning that this kind of fish is difficult to take care of before the fish died. because plenty of people dont notice it until then.