1. be very clear when talking pipe or hose whether you are talking about inside diameter or outside diameter. They use both measures.
2. don't sweat leaks in any join that's meant to be underwater.
3. use teflon plumbers tape on any screwon joint. Wrap the tape in the same direction as you screw on.
4. pipe connects to pipe with a slip on or a screw on type connector. Slip-ons are NOT fastened if always underwater. This includes your downflow. If slip-ons are used outside of...