They are using weight rather than length as the unit to measure the fish.
In a "music tank" fish stay together at one corner and don't swim around, by mean fish don't have any exercise. So, of course they will have a greater weight because they hold up fat inside there body.
And the opposite happened to the "No music tank", fish swim around and do have exercise. So, less fat in there body. Result in weight less.
So, if you want to have fat koi.
Than play music for them to listen.
So, i assume this work well for commercial fish farming. Because they sale their fish according to weight.
I am just sharing my opinion, so if i said anything that you think is wrong please forgive me.
Thanks.
sas