Water is often referred to as the universal solvent, meaning anything that enters the water will, in some form or another, in some amount or another, dissolve in that water and affect its chemical makeup. Any gas that is in contact with the water will also dissolve to some extent in that same water. Some of these compounds, elements and gasses - many different names are used, depending on how semantically precise one wants to be - that dissolve are beneficial, and others are toxic and noxious.

The key, therefore, is to ensure that only beneficial ones remain in solution and that all noxious elements are removed in some way or another. That is, of course, exactly the purpose of all filtration, and it is also the reason why the success of a tank really boils down to the relative quality of the water chemistry. The more efficiently these noxious compounds are removed, the higher the water quality will be and the better a living environment for your fish, corals and invertebrates you will be providing.

This chapter deals with some of these compounds, the ones most frequently encountered in an aquarium environment, especially by hobbyists who use tap, well, river, lake and other such types of water. Distilled, de-ionized and water that is chemically treated with, for instance, resins, is much purer, and will not contain these elements as a matter of course.

The listing in this chapter, and the analysis of each compound, is by no means to be considered exhaustive. Many other chemicals and elements can be, and are, present in aquarium water. A complete analysis would require more than one book just on that subject. Many beneficial ones are not discussed in this chapter either, but can be found elsewhere in this book, e.g. oxygen, dissolved oxygen levels, carbon dioxide, trace elements, and so on.

Natural seawater contains all known elements in solution, in varying quantities, many in such small amounts that they are measured in the parts per billion, and even lower. Many of these elements are present in the artificial salts that we use, and provide the basis for the environment in which the fish and invertebrates are placed. A fair number are not available, and are often added by means of supplements such as trace element mixes, specialized mixtures, individual additives, and so on.

Whereas you may think that such solves the problem of ensuring the presence of all the required elements and nutrients in the water, this is unfortunately not so for two main reasons:

• Trace element mixtures as well as other additives and supplements are not as complete as manufacturers would like them to be, and do not substitute efficiently for true seawater, as many elements are still missing. Upwards of 30 may not be present. Whether many of the missing elements are, in fact, really needed by the tank's inhabitants, has not been documented in hobby literature, at least not to my knowledge, and is often a matter of great debate.

To the manufacturer's credit it must be said that such additives cannot be made more complete because information as to what elements to add to make the mixes more efficient is scarce or non-existent. Unfortunately, in-depth research on the exact and complete requirements of invertebrates and fish is not as complete as we hobbyists would like it to be. Much progress can still be made in that area.

Fortunately however, as time goes on, many new discoveries are made. The recent introduction of a strontium and molybdenum supplement, and also an iodine supplement, by Thiel*Aqua*Tech is proof of that. Kalkwasser from the same company, an absolute must in reef tanks, is another example. Research is obviously not at a standstill, and new findings are acted upon as soon as they are known.

• All these compounds are present in an environment that is chemically very active: your tank. The chemistry underlying what happens in an aquarium and the biochemical and other processes that occur in that water, are far beyond the scope of this book, but result in many of these elements, amongst others, being:
• Removed from the water by the tank's inhabitants, during feeding, absorbing from the water and storing, metabolic processes, etc. sometimes rather quickly.
• Transformed into other chemical compounds that are not usable by the animals, but remain in the tank, either as dissolved or undissolved chemicals.
• Precipitated out of solution, often due to the high pH of a reef aquarium, and thus not available to the animals.
• Removed by chemical filters such as highly activated carbon and other types of chemical filters.
• Removed by protein skimming, foam fractionation, air scrubbing, denitrification, assimilation, the use of special compounds, etc.
• Stored by algae, especially certain type of macro-algae, (for more details see the chapter on macro-algae).

Artificial seawater not only contains all the additives that we purposely introduce, but it contains an even greater number of elements that we do not even know are there, because we do not test for them, or because no tests that can be used by hobbyists are available. Such elements enter the water in multitudes of ways, many of which you do not suspect, or are aware of. Here are a few:

• As by-products of chemical processes taking place in the tank, both chemical, biochemical, and redox processes.
• As leachings from rocks, etc. present in the aquarium. This is often the case when you place rock of unknown origin in the tank, e.g. red lava rock, rocks that came from aquariums previously treated with copper, medicine, and so on.
• As impurities of additives that you may be using, for example impurities in the salt, the foods, many grades of activated carbon, fertilizers, carbonate hardness generators, etc. Yes, even some of the latter contain more impurities than is good for your reef tank.
• As elements, chemicals, and compounds dissolved in the water used to perform water changes and top-off the tank. This is often particularly the case if you use tap or well water to top-off your tank and perform water changes, when such water is not treated beforehand.
• As pollution entering the water because it is present in the air.
• As pollutants that transfer to the water because of chemicals used in the daily maintenance routines of your apartment or house, of cooking, etc., referred to more generally as airborne pollutants.

For the majority, these elements, the ones that are unwanted, can be removed by using the appropriate form of filtration: chemical filters and foam fractionation and, in some cases, air scrubbing. Some can only be gotten rid of by performing water changes with water that does not contain such pollutants: either triple distilled water (H2O), deionized or reverse osmosis water (but you must use a reverse osmosis unit that has the right kind of membrane, and such units can cost several hundred dollars).

The fact that such can happen is also the reason why I totally disagree with anyone suggesting that water changes are not necessary. Some pollutants just cannot be removed by filtration. They must be diluted, and kept at non-toxic levels, by changing water on a regular basis (see chapter on maintenance as well).

This chapter, then, deals with some of these elements; desirable ones and non-desirable ones that are frequently found in aquariums that you should be aware of. It is certainly not an exhaustive list. The chapter also includes a short explanation for some terminology that you will frequently find in hobby texts in magazines and books.

A last note: macro-algae can store certain elements in their internal structures in concentrations far higher than the ones normally found in seawater. These can be several powers of 10 greater than the concentrations normally found in water. Keep this in mind when you have an algal die-off, or when you rip algae out of the tank when trimming excessive growth. As the stems are broken, fluids leach in to the tank's water. These fluids contain many chemical compounds, some of which may be toxic. Performing a 10-20% water change after doing so is, therefore, often recommended. The size of the water change depends on the amount of algae removed, and whether or not an algal die-off had, in fact, occurred.

Important Note: Some of the remarks made in this introduction may seem trivial to you because in many cases we are talking about such small quantities of these chemicals. Such may very well be the case, but keep two important factors in mind: