Actually it's not half, or better yet, keep in mind I only watched the italian version and that sometimes the numbers are different between versions, so I don't know if that also goes for the english one, but in the italian version the tanks are 1 meter vs 60 cm deep, that's 60% of the deepest one, so with 14 fish in the big one it would be 8,4 in the small one, so I guess they rounded up to 9.