Knowing the accurate size of your fish is useful, in keeping accurate observation regarding the health of the fish, in discussion with other reefers on line, in selling your animals and many other things that we addicted reef keepers do in our day to day caring and boasting about our fish.
Over the years, I often surprise at how far off my estimate of my fish size are so inaccurate. Daily looking at the fish I have an estimate of the fish size. Once in a while, one of my wrasse would sleep at one of the back corner of my tank. Having my tank in a fish room, I can get up really close to the fish that sleep there. In these situations, the fish often looks much larger. Whenever I have these opportunities, I often grab my ruler and measure the size of the fish. I am often surprise that the fish is often at least 20%+ larger than what I think his size is by just looking at him from the front of the tank.
Lately, I have been doing things a little different. Knowing that I have the tendency to underestimate the size of the fish, I tried to accurately measure the size of some of my fish and use these measurements to estimate the size of other fishes in the tank as they swimming about and come in close contact with each other.
One way I find that I can accurately measure the size of my fish is tape a 3X5 index card on the front glass of the tank. Then I would take picture of the fish using a telephoto lens from across the room, as far from the tank as I can as the fish swim nears the index card. The closer the fish to the index card the better. The optic of camera is such that there are distortion regarding the size of the objects relative to the distance that object is to the camera. A picture of 2 objects of the same length, but object A at 1 foot and object B at 2 foot from the camera will show object A have twice the length of object B in that picture. Because of this reason, having the fish next to the index card and the camera from 10 feet away from across the room, the size distortion is neglectable.
Below is a demonstration of this method that I used to measure the size of my fish.
My 4 yo Purple tang, I estimated to be just about 4 inches but using this method, I measured him at just over 5.5 inches.
Powder Blue tang at 7 inches