Bro why dont you catch it

Why would I?

So it wouldnt despawn

It's just a normal Goldfish.
If it was a Golden Goldfish then I would've caught it.

I dont say use it as a bait or something
I think you should build an aquarium for it

You can't use goldfish as bait.
And the best you could do for an aquarium with a goldfish is a goldfish bowl.

Then do a bowl

Nah