I'm pretty sure you can't "pick up" a fish tank and carry it around.
What you probably did was inadvertently placed it down after you bought it.
Go back to Willy's, get in the same position as you are in your screenshot, make sure you have an open slot in your backpack (preferably one you can see), then press down on the fish tank. It should pop into the empty slot. Then choose another slot in your backpack so the fish tank is no longer highlighted, otherwise you'll chance putting it down again.