An air lift is a very simple device to generate water circulation where you don't want to use a powerhead because of the fragile life forms there which might be harmed by a powerhead or pump impeller.

I used six of them on an acrylic sheet to create a race track circulation in a brine shrimp tank.
(Click here for a picture.)

I got the plans from The Plankton Culture Manual by Frank H. Hoff & Terry W. Snell. This is a very good book if you are interested in raising any type of plankton.

All that is required to make an air lift is a piece of PVC pipe with one end cut at a 45 degree angle and a PVC elbow on the other end with a 3/16" hole drilled in the top corner of it.

Insert a piece of rigid airline in this hole and push it down almost to the high side of the 45 degree opening.

Set the length of the tube so that the elbow is half out of the water.

Hook an air pump to the top of the rigid airline and adjust it so that you get a slow stream of large bubbles.

This is a nice addition to a refugia.