<a href=showthread.php?s=&postid=6888506#post6888506 target=_blank>Originally posted</a> by hulley
Basically a big clear tube (kinda like a skimmer) and water enters the tube from the bottom, as the water rises to the top, the water will pass through fine sand that colonizes the "good" bacteria and biologically filter the water (like live rock), then the clean water flows out the top. Very simple and effective looking idea. Supossedly with no nitrate problems like bioballs have. Marinedepot.com sells them if you want to see what they look like. According to my Marine fish encylopedia, they are very effective at biological filtration without taking up alot of space.