To cut down the free surface movement
1) If using soil pipe (or drain pipe ) it only really needs to be about 3/4 of the tank high.
( when the tank is full the water dosen,t swill around )
If you get to look inside a ionic system you will see.
2) The drainage pipe is also used for draining land so a farm supplier might have it . I have put this in fields and I think they punch the holes into it so it will need a good rinse to remove the waste plastic.
Ray