There's nothing weird about that. That's how the game works. The little "tank" you see in the shop is used to display a fish that has just been sold. The iso tank displays only when you're looking at one of the three fish tanks. The selling tank (Tank $) can be used for breeding and growing fish, just like the other two tanks. The only (and important) difference is that any adult fish in that tank when you switch to the shop (by tapping the Sell Fish button) will be for sale. You could inadvertently sell fish that you want to keep unless you are aware of the distinction.

You might want to check out this page - it explains a lot of the basics of Fish Tycoon.