Have you talked to her about it? I'd maybe send her an email saying something along the lines of, "I couldn't help thinking about your goldfish the other day. I was wondering if you knew that your vase is too small for him? I know the petshop probably said it was fine, but it's not ideal...[insert sympathetic, non-judgemental explanation of what a fish suffers in a space that's too small, tell what a goldfish actually needs, ask if she's considered rehoming it as a result]."
People mistreat their animals so badly out of pure ignorance. :(