Thanks for the comments.
I was a bit worry about the grow rate, as in Asia here, many people has manage to grow the same batch of fish to 60+cm now.
The sumi has became lighter, and a dealer told me that it is quite normal for momotaro sanke, whereby in tosai, strong sumi, nisai and early sansai, weak sumi, and then when sumi return strong, it will stay there.
So far....I can only listen to believe, since this is my 1 batch of Momotaro fish.