You won't be annoying! Just say how a lot of times these pets are misunderstood--we shouldn't just buy something because it's 'for bettas'.. Think about commercials, do we believe every single commercial 'proven' to make us lose weight, gain muscle, get smarter, etc. etc. whatever?
Just say how much prettier she can decorate the nice big tanks, how much less time she will have to spend cleaning them, and how much more active they will be!