Lammikoi.
Accually, 4-6" by 3-4 month old is normal size and it would be possible if the dealer received a shipment from somewhere in a warmer region like Hawaii or South Asia...... but I think these fish that you saw a few days ago were not imported from Japan. March/April is still too cold to breed and new born tosai will be available for sale by the end of Aug - Sept.
Shimegai is a technique to maintain the size of the small koi, for example 8" 3 years old, and finish the color by color food. Those koi are very strong for koi show compatetion.
Jumbo tosai could be reached over 20" by March/April. It's not so many but almost 24" of the super jumbo tosai is available too. That's WOW
