Is this a valid riddle in Mandarin?
If he elaborates, the whole point of asking the question would be gone.
I suppose only Cantonese speakers would see the point as it is a play on  魚 and 如 which in Cantonese sounds exactly the same, but pronounced in Mandarin as 魚, (Yú), and 如, ( Rú)
The poem's correct title is 美人如玉劍如虹. The riddle is substituting 魚, for 如 as they sound exactly the same in Cantonese.
Will it work in Mandarin? A Mandarin speaker with no knowledge of Cantonese would not get it. I hope I got that right?