Aquarium like camera

https://docs.unity3d.com/Manual/FrustumSizeAtDistance.html

With a little bit of tweaking and adjusting, you can ensure that the closest walls (i.e. left/right/top/bottom) of the tank hit right at the edges of the screen at a specific distance. From there, you'll have depth beginning at your choice of distance from the camera.

With that in mind, you would be able to position boundaries at each corner relative to the screen.

 // C#
 // For a script attached to the main camera
 
 // These Transforms (ideally, unit cubes) are named
 // based on the idea of looking in
 // from the missing side
 public Transform leftWall;
 public Transform rightWall;
 public Transform floor;
 public Transform ceiling;
 public Transform backWall;
 
 public float tankDepth = 5.0f;
 public float distanceToTank = 10.0f;
 
 // Based on a unit cube (1x1x1)
 public float wallThickness = 1.0f;
 
 void Start()
 {
     // camera parameters
     Camera cam = GetComponent<Camera>();
     Vector3 camPos = transform.position;
     Vector3 up = transform.up;
     Vector3 forward = transform.forward;
     Vector3 right = transform.right;
     
     // Camera frustum information, based on
     // Distance To (Fish) Tank
     float frustumHeight = 2.0f * distanceToTank * Mathf.Tan(cam.fieldOfView * 0.5f * Mathf.Deg2Rad);
     float frustumWidth = frustumHeight * cam.aspect;
     
     // Sizing keys for sides
     float wallDepth = distanceToTank + (tankDepth * 0.5f);
     float halfThickness = wallThickness * 0.5f;
     float doubleThickness = wallThickness * 2.0f;
     float halfHeight = frustumHeight * 0.5f;
     float halfWidth = frustumWidth * 0.5f;
     
     // ceiling and floor are extended beyond the
     // frustum width to include the width of
     // the left and right sides
     ceiling.position = camPos + (up * (halfHeight * halfThickness)) + (forward * wallDepth);
     ceiling.localScale = new Vector3(frustumWidth + doubleThickness, wallThickness, tankDepth);
     ceiling.rotation = transform.rotation;
     
     floor.position = camPos - (up * (halfHeight * halfThickness)) + (forward * wallDepth);
     floor.localScale = new Vector3(frustumWidth + doubleThickness, wallThickness, tankDepth);
     floor.rotation = transform.rotation;
     
     rightWall.position = camPos + (right * (halfWidth + halfThickness)) + (forward * wallDepth);
     rightWall.localScale = new Vector3(wallThickness, frustumHeight, tankDepth);
     rightWall.rotation = transform.rotation;
     
     leftWall.position = camPos - (right * (halfWidth + halfThickness)) + (forward * wallDepth);
     leftWall.localScale = new Vector3(wallThickness, frustumHeight, tankDepth);
     leftWall.rotation = transform.rotation;
     
     // the back wall will extend to fit everything
     // else for convenience in this example
     backWall.position = camPos + (forward * (distance + tankDepth + halfThickness));
     backWall.localScale = new Vector3(frustumWidth + doubleThickness, frustumHeight + doubleThickness, wallThickness);
     backWall.rotation = transform.rotation;
 }

With this setup, all you have to tweak are the tankDepth and distanceToTank values and the tank will resize itself accordingly.

^{Edit: Fixed some formatting}