Fish do not really realize that they are in a tank.
Although fish do have awareness of their surroundings, boundaries and limitations, but fish don't understand the abstract concept of being in a fish tank.
Fish learn the layout of their environment and recognize their feeding routines and can also feel stress from being confined in the tank, but fish lack the higher level of cognitive ability to conceptualize their own captivity in the fish tank.
Fish are also fully aware of their water quality and conditions and become stressed in small or poorly maintained fish tanks.
However fish don't conceptualize the water itself, just as us humans don't constantly perceive the air around us.
Fish also learn their environment, including on where to hide and when to expect their food.
Fish demonstrate that they do understand the specific layout of their fish tank, instead of just the concept of being trapped in a fish tank.
Fish also sometimes engage n what is known as "glass surfing" where the fish swims up and down the glass of the fish tank, which can indicate the fish is bored, stressed or is a new fish that is attempting to understand their boundaries.
When fish are in a fish tank, the fish also understand that they are indeed in a limited space because they encounter the physical barriers like acrylic/glass, which prevents the fish from traveling any further.
So while the fish may know that they are in a fish tank, the fish is sentient enough that they can react to their own environment and recognize their owners and experience stress.