Why did the Eunuch ban the consumption of fish, and get agitated when the Crown Prince get presented with two bass (very ordinary fish) and then tipped the chefs who cooked it?

The reason this is missed is because of the meaning behind the Eunuch's name in Chinese.

The Eunuch's name is Yu Chengan 鱼成安 -> and the surname "鱼" is literally fish.

It's implied during the story that the Eunuchs hold actual power in the court.

He's furious that the implication is that the Crown Prince has cooked and consumed his namesake, giving him disrespect. The Crown Prince tipping the chefs also suggests that he approves of this, that he wants the power back.

In an ordinary court where the Emperor held all ruling rights, a Eunuch would never have the power to ban the consumption of fish amongst royalty, as he is part of their serving class.