Among the Gu Family's three shops, the Spirit Fish shop brought the least income even when their businesses weren't being suppressed by Liu and Song families.
The reason was simple: This was a world where everyone with some means pursued cultivation to become stronger as only by doing so they had a chance to live their lives freely.
Since this was the case, people tended to spend most of their money on cultivation resources. As for food and clothes, as long as they didn't go hungry or their clothes weren't completely unusable, they wouldn't spend much on them.
While the name of their shop had 'Spirit Fish' in it, they rarely sold spirit fish as they were very few in number in the river and it was extremely difficult to catch them.
Thus, usually they only sold normal fish. Since people didn't want to spend much on normal food, the shop obviously didn't earn much profits.
