A protein skimmer is the primary filter in a Jaubert Filter, removing a majority of the ammonia generating DOC's (Dissolved Organic Compounds), which are created by tank critter detritus, uneaten food and other decaying matter in the tank. Understanding the principles of foam fractionating (protein skimming) will greatly assist you as you design a new system or upgrade an existing one. "What Is Protein Skimming, and How Does It Work?", by Don Carner, describes skimming, is written in understandable language and is an excellent read.
The Live Sand actually serves two purposes in a Jaubert type filter:
- Biological filter media, providing surface area for beneficial nitrosoma and nitrobacter nitrifying bacteria to grow on.
- Gas barrier, keeping CO2 in the plenum and O2 the upper level of the Live Sand, allowing the filter to function properly.
A plenum is little more than a vacant space in the substrate, harboring anaerobic bacteria, which converts nitrates into nitrogen. In order for the anaerobic bacteria to function, the plenum must contain carbon dioxide and be devoid of oxygen. If you remove the up tubes and their associated air tubes from an Under Gravel Filter you will essentially have a basic plenum. DIY Under Gravel Filters are inexpensive and easily built in a few minutes.
The original Jaubert filters included a deep bed (4"-5") of live sand over the plenum, which essentially seals off the plenum from the oxygen rich water in the rest of the tank. The upper level of the live sand provides a home for the nitrifying bacteria, which converts the generated ammonia into nitrites, then nitrates, as well as a home for all of the sand stirring critters that keep the substrate clean.
Over the years, the Jaubert system and the use of Deep Sand Beds have been the subjects of a number of heated debates. Dead spots in an aquarium are a prefect place for detrimental bacteria to flourish. These bacteria can produce infections on fish which look very much like saltwater velvet (Ick), but does not respond to treatments normally applied for Ick.