Gold Goldfish

Unlike the gold critters on land, this one lives in water, so the usual problem is reaching it before it drifts somewhere you cannot swing. Any Bug Net will take it, and a Lifeform Analyzer is what tells you there is one worth chasing in the first place. It dies to almost anything, including a careless swing of your own weapon.

It is worth the same whether you sell the critter or the furniture made from it, so the decision is purely whether you want the money or the fish bowl. Caging it is permanent unless you run the furniture back through Shimmer. Note that the tooltip implies more than the game delivers: the Terrarium will not accept it, and Bottled Water is the only other container it will sit in.

A gold goldfish is a joke about the word already containing its own color, which is the sort of thing the tooltip is happy to point out at your expense.