Biologic evolution

It tried variously whether it was the best when sand in the water
tank was very done.
It is hardness of sand that thought most.
Because it was possible to supply it with the pump, oxygen was not
thought so much.
I thought that it was the best that the hardness of sand was soft.
When the amount of sand is large, sand becomes hard.
The shellfish cannot dive in sand when the amount of sand is made few.

And, the idea method :.
Thing to take amount of sand enough without sinking sand under water
tank

It made it to how to put sand in this photograph.
The filter was paved in the bottom of the water tank, and sand was
put on that.
Sand is soft to the extent that hydraulic pressure doesn't hang.
It will breed for the time being by this style.