Fill fish tank 2/3 full with water

Suspend the ball from the ruler placed over the fish tank.  The ball should be toward one end and about centered in the water.

Position the sight tube so that when you look through it down into the water, the ball is centered.

Now carefully substitute the green laser pointer for your eye.  It should fit snuggly in the end of the tube (only one end of the tube is bored for it).  The laser beam should now be centered on the ball.

Note the path of the laser beam from the side of the tank.

NOTE: The current green laser pointer works well.  Other laser pointers may not have the beam parallel to its body, and the beam will not emerge from the end of the tube.

Now insert the black carbon fiber tube through the sight tube.  The carbon fiber tube does not follow the line of sight but ends up above the ball.