In no other habitable place on earth are so many conditions so stable and so enduring as in the ocean. This stability of the sea as an environment of life is the fundamental cause of difficulties that most aquarists have in keeping marine fishes and invertebrates. Because these animals evolved in a world that has remained practically constant in temperature, osmotic pressure, alkalinity, and chemical composition, they have developed few mechanisms to isolate themselves from these environmental factors and little tolerance for change. Once sea water has been removed from the ocean, it begins to change and to lose some of its capacity to support delicate marine life. This is directly and indirectly the result of its longer being a part of water that is infinitely large. The best you can do is to slow down this deterioration.
Sea water is not only an inorganic complex. An ever-increasing number and variety of organic substances are being found in it. Among the various types of organic substances that have been found in sea water are enzymes, vitamins, pigments, amino acids, antibiotics, and toxins, most of them produced as external metabolites by plants and animals in the sea. Some of them are important in the culturing of larval forms and macroscopic invertebrates.
The sea contains a widely distributed microscopic fauna and flora consisting of protozoans, algae, fungi, yeasts, and bacteria which are important in sea water. Although sea water has well-recognized bactericidal effects, it also provides an excellent medium for some bacteria, and when enriched with organic matter, may contain billions per cc.
An important factor of bacterial control in marine aquariums is cleanliness, achieved by avoiding overfeeding and overcrowding and by practicing systematic cleaning procedures. In addition, four methods of reducing the number of bacteria have been used: filtering, storage in the dark, ultra-violet treatment and addition of antibiotics.
The fundamental difference between the behavior of small standing freshwater aquariums and those containing sea water lies with their microbiology. The real balance of a marine aquarium consists in the relatively stable condition of its microbial population.
Changes occurring in sea water aquariums after long use
The outstanding changes occurring in sea water that has been kept in circulatory systems for months or years are two: a permanent lowering of the pH and an accumulation of nitrates. In nature, the pH of sea water seldom exceeds 8.4 or drops below 7.5. It is regulated by a series of chemical processes involving carbon dioxide, carbonic acid, sodium bicarbonate, and sodium carbonate which forms a buffer.
Sea water dissolves about 20% less oxygen than fresh water. Marine fishes that are exposed to sea water supersaturated with air frequently develop gas-bubble disease in which bubbles, most of nitrogen, appear under the skin, especially on the fins, and within various vital organs, causing death.
The need for constant treatment of sea water in order to keep it satisfactory for marine life is apparent. To measure pH, chemicals called indicators may be used. These are added by drops to a small sample fo aquarium water and change color according to the pH. Thus, the commonly used bromthymol blue is yellow when at a mildly acid pH, green when neutral and blue when alkaline, covering a useful range of about pH 6.0 to 7.6. For sufficiently accurate work color charts are supplied in steps of 0.2 or 0.3. Another method is a comparator paper which is dipped straight into the tank and compared with a chart afterwards. These are not always reliable after long storage, and for a really accurate pH measurement, electrometric methods are used.