Are there similar metaphors to “It’s no use for a dojo (loach) to behave like a goldfish”?

There's also

The leopard can't change its spots

(which is along the lines of "It's no use trying to change who I am")

and, more derogatory,

You can't make a silk purse out of a sow's ear

("You can't make a high-quality item using shoddy materials")

Keep your feet on the ground (and your head out of the sky).

This isn't a formal idiom, but it does draw on a formal fable and a metaphor that is commonly utilized in English:

"There's no point in a country mouse putting on city mouse airs"

This might work as an acceptable substitute. It goes back to Aesop's fables, where the country mouse is a humble, diligent creature with a comfortable, largely peaceful life, while the city mouse is a dandified, lazy creature of many comforts, but who must also suffer considerable danger and terror to enjoy them.

The problem is that this sort of simile is very easy to construct in English, and so many people just do it on-the-fly. Idioms are usually phrases that succinctly describe ideas that are difficult to express otherwise, but casual metaphors like this are quite common in English.

Not really a metaphor, but "Slow and steady wins the race" has kind of a similar connotation.

I interpret “I’m a dojo. It’s no use for a dojo to behave like a goldfish.” as saying that "I know my limits. It's no use trying to be something other than what I am."

In that sense it's self-deprecating, but if you wanted to use it for something or someone else — it's no use trying to make something appear better than it is — then there's the recently famous

"It's no use putting lipstick on a pig."

(It's still a pig.) Be careful: this is usually dismissive and derogatory.