Do you ever see a picture of a koi and you cannot discern the size of the fish? I certainly do.
Featured on this blog is a koi that is a jumbo Shiro Utsuri. This koi is Jumbo and is so big that when I try to get a close up picture she would not fit in the frame. This makes it difficult for me as the photographer to display the size of the koi and the quality as well. Some dealers are starting to place objects in the bowls to illustrate the size of the koi. This seems to work to some degree but overall it depends on the behavior of the koi. I have taken pictures of "well behaved, photogenic koi" and I have taken pictures of koi that are hugging the sides of the bowl, or like others that cannot sit still long enough to get a quality shot. For those pictures I apologize, but sometimes you get what you get. This is really due to the length of time one has to take a picture of each koi.
We are using a professional camera with a 25mm lense, a polarized lense in front of that, a hood, and a mounted light without a flash. I do this so that when I take the pictures I can have a consistent light source. I found that when a flash comes into play the light scatters and you have no idea what will be in the shot when we fly half way back around the world. This can be a problem. In order to try and solve these problems I am going to make an attempt to switch to video. This way people can observe the koi swimming and they may be able to get a good idea of the size, I will make an attempt to place and object in the bowl as well.
I hope this helps to explain the experience of photographing koi.