Fish aren't sold unless you click the Sell Fish button and you see the Fish Store. It's safe to keep babies in the sale tank, but it's risky to keep adults there unless you want to sell them - all it takes is accidentally clicking on Sell Fish and they can be gone. Once a fish is sold, you can't get it back, so you'll have to make another one.

You could buy some more eggs to get one, or, if you have a Silky Shark and a Silky Beta, or a Silky Stickfish and a Silky Fatfish, you could breed one.