The most effective size cube was the 1 cm cube.

Since it is the smallest cube, there is less volume for the base to travel through. Surface area, temperature, and concentration are important factors that affect the rate of diffusion.

Large surface area is helpful towards diffusion, but increased volume is not. The larger the volume, the slower the rate of diffusion will be.

Bacteria are single celled, and cells only grow up to a certain size. Fish are multi cellular and contain many cells, so they can grow to a much larger size.