The purpose of all filtration is to remove unwanted compounds from the tank's water, those compounds that when dissolved make the environment less suitable for keeping corals, fish, invertebrates, and any other life forms that you may be planning to add to your aquarium, now or at a later date. As already indicated in the previous chapter, as soon as animals are added to the aquarium, nutrients are depleted and pollutants are added. Both these processes occur continuously, 24 hours a day, day in, day out.

We will deal with the replenishment of the depleted nutrients later; at this stage we are concerned with the types of pollutants that are added, and how they can, or should, be removed before they can cause damage to the animal life. Each type will require a special form of filtration, geared to dealing with specific elements and compounds that we wish to remove from the water.

What compounds are we talking about? There are numerous such compounds, so many that it does not even make sense to list them all. A number of them are, however, well known to you because they are often mentioned in aquarium literature:

• Nitrogen intermediate breakdown products such as ammonia NH3, ammonium ion NH4, nitrite NO2 and nitrate NO3.
• Organic material, both dissolved and not dissolved. Proteinaceous matter from food, small life forms that died, and so on.
• Yellowing matter, Gilvin, Gelbstoff. Albumin.
• Ortho-phosphate. Organic phosphate. Other forms of phosphate e.g. hexa-meta-phosphate.
• Chemical pollutants: fumes, nicotine, cleaning product vapors, perfumes, sprays, polishing products used in the house or apartment, etc.
• Heavy metals that enter the aquarium through water changes, leachings from rocks, etc. Examples include copper, zinc, and others.
• Pesticides that may come in with tap or well water, granted in the parts per billion range, but if they build up over time they will cause problems.
• Amines, nitrosamines, Indoles, scatoles, Phenol, Hydrocarbons, etc.

Most, if not all, of these compounds can be removed rather easily from the aquarium water, providing the hobbyist installs the appropriate type of filtration for each type of pollutant. These forms of filtration may be known to you by the following names:

• Biological filtration
• Mechanical filtration
• Chemical filtration
• Foam fractionation, or protein skimming
• Ion selective filtration
• Air scrubbing

Because the relative success of your aquarium, or said differently, the survival rate and quality of appearance of the animals in the tank, depends for 99 percent on the chemistry of the water, it is most important that reef keeping hobbyists pay very close attention to all of the above types of filtration. All are covered, in detail, in this book.

Installing only part of the required types will result in lowered water quality levels and an aquarium that does not look as good as it could, meaning an aquarium where the animals are not in as good a shape as they could be if more filtration were used.

Anytime you decide to make a trade-off, and forego some form of filtration, or use a process that is less efficient than the one we suggest, you are exposing yourself to problems of many kinds. Usually the first signs of such problems will be the outbreak of parasites on your fish, and less healthy looking corals and invertebrates. Trade-offs may save money in the short run, they will, however, end up costing you more when you factor in the cost of replacing fish and corals. Be aware of it and think twice before making such decisions.

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