What in the world is a Jumbo Tosai?
It sounds like some kinda new frozen food, does it not ? Tosai is a Japanese term for “ Koi of the season”.

Over the last few years Momotaro, and others have “ come up' with these new miraculous little fish that, instead of overwintering in cooler waters, with slower growth rates, are pushed in warm, high quality, water with abundant feed. Nothing more to it !

It's supreme marketing to get you to buy a fish... a “miracle fish” that might only cost $100-$200 dollars but the miracle fish, of 12-14-16 inches at nine months is sold to you for SEVERAL times that amount in order for you to think that you, yourself, can “make” a thirty inch Koi in record time!

Folks. It's rubbish! I can, and do produce “Jumbo Koi” from all sorts of different bloodline Tosai that I purchase. I have the water resources and knowledge to do it.

I can take the “lowly” normal 5-7 inch Tosai that regular folks purchase, and BLAST it into a Jumbo quite rapidly often having it surpass the growth of the “Marketed Jumbo” within 18 months, IF I choose to. I can also color up that lowly tosai with color food to make him look like a 5 inch Grand Champion. I do neither.
Jim Reilly, a friend , Koi judge and well known advanced hobby guy/amateur scientist said it best. Many of you have seen his writing as “ JR” or “JPR”
Thanks, Jim.
"Here are some 'hard truths'
1) Koi are the ultimate 'cut flower'. They improve ( tategoi), they peak ( finished) and they decline( past sell date). We, conversing here on this thread know this well. But every year, people spend small fortunes on fish thinking they are living art, like oil paintings, and that they will preserve these looks forever. Usually 'forever' is 1-5 years.
2) Koi that are pushed, grow no larger than if they are 'done well' in a seasonal setting. And there has long been evidence and feedback that these techniques can produce unhealthy koi in the end. Allow me to open the top of the hamper and expose some pieces of dirty laundry. The fish of Hiroshima need to be ' hardened off' in the old days as they were as weak as kittens when shipped abroad. One of the breeders already mentioned struggled with a strange virus for years on and off.
And finally Maeda himself said in an interview on this subject that in the end, mudpond/wintered fish and his hothouse fish will be the same size but outdoor/wintered fish have greater vigor and a look of a 'wilder fish' ( he meant this as a compliment).
There is nothing wrong with giving fish a 'head start' in life. Strong robust fish can be a joy to own. But the 'implied' message to the enthusiastic beginner is that the fish will continue to grow at this pace and that a 'giant' Koi has just been obtained. This is not true. Koi are indeterminate growers within a programmed genetic limitation.
Well said, Jim.