stream rock question

[StrategicReef]

I want to know how the flow get pulled into the rock, is the bottom of the rock very hollow, is it easy to aim it up and down?

Also is each rock unique and approximately what size are they?

 

[rvitko]

Each rock is somewhat unique as they are hand made of ceramic. They are approximately 10" by 10" by 6" They are open underneath it is basically a cover with 3 legs so they have a lot of open space underneath and in the back.