"Ask the canal" means "ask it". In this place, "it" means "square pond". The poet didn't say how deep the square pond was. The third poem highlights the word "Qing", which already contains "Shen". Because if the water in the pool does not have a certain depth, even if it is "clear", it cannot reflect the modality of "the sky is high and the clouds are light".
If the poet grasps the "deep" and "clear" pool water, he can reflect the characteristics of "the sky is light and the clouds are light". But the poet didn't stop there. He raised a further question. "Ask" the "square pond" "Is it that clear?" Ask why it is so "clear" that it can reflect "the sky and clouds". There is no way to answer this question by looking at this square pond in isolation.
The poet then opened his eyes and looked from a distance. Finally, he saw the "source" of "Fangtang" and found the answer. It is precisely because "Fangtang" is not passive water, it has inexhaustible "source" and continuously delivers "living water" to it.
This "square pond" will never be exhausted, stale, dirty, deep and clean because of the continuous input of "flowing water from the source". "Qing" can not only embody "sky and clouds", but also embody such detailed modality as "sky and clouds" and "* * *". This is the image and ideological significance of this little poem.