Strong currents coupled with readily digestible protien and calories create a growth metabolism. A physically active lifestyle in humans (strong currents for Koi) stimulates metabolic rates resulting in better digestion/assimilation of protiens, etc... so long as easily burned calories are available. I'll bet if you were to dissect koi from both of your friends ponds you would find a tremendous difference in both skeletal and muscle mass as well as a big difference in muscle texture. Kind of like the difference between wild Trout and Salmon(Bright, firm, flakey meat) vs. pond raised (pale, dull, and mushy).