The most important physical and chemical cycles operating within the saltwater aquarium are biological filtration, gas exchange, and the day/night cycle. Without biological filtration, an aquarium requires water changes so frequently as to be impractical. Life in an aquarium cannot exist without the exchange of oxygen and carbon dioxide at the surface. Photosynthetic organisms require both light and darkness for their survival, and the alternation of light and dark regulates the metabolism of both fish and invertebrates. Good aquarium design and regular maintenance take care of all these requirements.
22 Saltwater Aquarium Models
22 Saltwater Aquarium Models
The chemistry of the surrounding seawater varies little from one coral reef to the next. Therefore, only one set of parameters is needed for all saltwater aquariums that display reef fishes and invertebrates. Some authors recommend maintaining the temperature at a slightly higher range than recommended here. Otherwise, broad agreement exists regarding the numbers presented here.
Salinity: 35 parts per thousand (ppt), 36 ppt for Red Sea Specific Gravity (at 75°F): 1.0260 pH: 8.0-8.5, optimum 8.3 Alkalinity: 7 Meq/L
Dissolved Oxygen: 6.9 mg/L (= 100 percent saturation) Calcium: 380-420 ppm Iodide: 0.06 ppm Phosphate: undetectable
Nitrate: see page 36 for a discussion of nitrate concentration and what it may mean Nitrite: undetectable Ammonia: undetectable
Fish excrete their wastes directly into the water. Under natural conditions, fish population density, considering the total volume of water surrounding a coral reef, is much lower than that of even the largest aquarium. Dilution, therefore, immediately counters fish waste pollution in the ocean. Additionally, in a short time natural processes degrade the wastes into simple compounds that can be taken up by algae or utilized in some other ecological process. When we establish an aquarium's artificial ecosystem, we must harness these same natural processes to promote the survival of our fish and invertebrate display. The totality of these processes as they occur in an aquarium is biological filtration. Biological filtration is the detoxification of wastes by beneficial bacteria known as nitrifiers or nitrifying bacteria. Coating every available surface that lies in contact with oxygenated water, these organisms chemically convert ammonia (the primary component of fish waste) into nitrate (a relatively harmless compound taken up by photosynthesizers). Biological filtration, or biofiltration, readily develops in the aquarium. All that is required is an ammonia source (fish) and the right kinds of bacteria. The latter are automatically transferred along with fish or any other item taken from the reef or from a previously established saltwater aquarium (the dealer's inventory system, for example). Within a month, nitrifying bacteria will colonize the aquarium system sufficiently to process a moderate amount of waste. The gradual development of biofiltration capacity prompts the widely offered recommendation always to stock the aquarium slowly, over a period of several months. Within six months to a year, the population of
Caring for an Aquarium 23
beneficial nitrifying bacteria will have matured completely, and biofiltration will be adequate to permit fish to be stocked at full capacity indefinitely.
Though biofiltration is a totally natural process, most aquariums are outfitted with some kind of filtration system. Designed to maximize biofiltration capacity, aquarium filtration equipment may employ a variety of techniques to increase the surface area available for colonization by nitrifiers. The bacteria refuse to carry out the desired chemical transformations when they float freely; they need to be stuck to a solid surface. Thus we have rotating "bio-wheel" devices, "wet/dry" systems, and "fluidized bed" technology. All these filtration methods provide extremely efficient biofiltration, converting all the ammonia generated within the tank to nitrate in a short period of time. Aquarium system design sometimes focuses on biofiltration to the exclusion of other important factors because the aquarist is often seen as trying to squeeze the maximum number of fish into the minimum number of gallons. Although you could buy a highly efficient filter system and have the tank teeming with fish, you would be inviting disaster, nearly guaranteeing it, because you would exceed what I like to call the true carrying capacity of the system.
We can debate all day about carrying capacity; that is, how many fish of what size a particular aquarium can support. If by support we simply mean "adequately detoxifying the ammonia waste produced," we can bump up the number of fish to high population densities indeed. Consider how many fish might be packed into a dealer's inventory system, for a case in point. Ten saltwater fish in a fifty-gallon tank would not be considered unusual. For the home aquarium display, biofiltration is not the whole story. We must think about the long-term success of an aquarium whose residents will be there for the rest of their lives. Fish and other organisms need what I like to call ecological space. A given species may need swimming room, a minimum number of companions of its own species, or a certain amount of water movement to really thrive. The ability of the aquarium to provide for these needs as well as for basic waste removal is a measure of the true carrying capacity. Taking into account not only waste removal, but also the need for ample oxygen, swimming room, and benign social interactions, ecological space must be allotted in the process of designing the aquarium. Care must be taken not to exceed the true carrying capacity of the system.
Regardless of its design, every aquarium needs regular partial water changes. I suggest removing 10 percent of the water weekly and replacing it with freshly prepared synthetic seawater. Depending upon your schedule, you might elect to change 20 percent every two weeks or 40 percent monthly, but the aquarium will look better and its inhabitants will appear more vibrant with more frequent, smaller changes. Partial water changes not only dilute nitrate that accumulates as a result of biological filtration, but also removes other forms of pollution that can harm fish and invertebrates.
Gas exchange is crucial. The water must continuously contain sufficient oxygen and must be continuously rid of carbon dioxide. While photosynthetic organisms, algae, and some invertebrates absorb carbon dioxide
24 Saltwater Aquarium Models during daylight periods, at night this may not be enough to prevent the accumulation of CO2. Carbon dioxide dissolves in water to produce carbonic acid, which drives down the pH and can inhibit critical respiratory processes in the fish. In sufficient concentration, CO2 is lethal. Merely agitating the water at the surface facilitates most, if not all, needed gas exchange. All saltwater filtration systems require water movement, and this usually creates plenty of surface action. Problems sometimes do occur when accumulated debris clogs the filter and causes it to slow down, and the resulting change in flow rate goes unnoticed. Many aquarists add immersible water pumps, known as powerheads, to increase both surface agitation and movement deeper in the water.
Gas exchange may be inhibited, regardless of the degree of water movement, when too little surface area exists for the volume of water in the tank. A tall, narrow tank has considerably less surface area per gallon than a shallow, broad one. Consider the following comparison between two commercially available sizes of tanks:
A fifty-gallon breeder tank (36 x 18 x 18 inches) has 4.5 square feet of surface, or a ratio of 0.09 square feet per gallon. A seventy-seven-gallon show tank (48 x 12 x 24) has only 4.0 square feet of surface, or 0.05 square feet per gallon. That is roughly half as much surface for 1.5 times as much water volume. To maintain the oxygen content of the water in the larger tank, plenty of water movement is required.
Gas exchange must be taken into account in developing an aquarium design. A tall tank may be dramatic in appearance, but it needs to be correspondingly broad (most aquarium shop owners would say deep) to provide adequate surface area.
Oxygen enters the aquarium and carbon dioxide escapes it via the water surface, but water must circulate within the tank so that oxygen remains constantly available to the inhabitants. similarly, carbon dioxide must not accumulate. Photosynthesis can account for significant oxygen production and carbon dioxide removal during the daylight hours. At night, photosynthesis ceases, and organisms that were formerly adding oxygen and removing carbon dioxide are now doing the opposite. In the dark, surface exchange must be relied upon. Even if the filter turnover rate meets the 500 gallons per hour standard suggested above, you may need to provide additional water movement via powerheads in order to facilitate adequate gas exchange.
with a pH test kit, you can determine if you have enough water movement. without sufficient exchange, carbon dioxide accumulates in your aquarium, reducing the pH. Remove a gallon of water to a bucket and aerate it vigorously overnight. The next morning, test the pH of both the tank and the bucket. if the pH of the bucket is 0.2 or more pH units higher than that of the tank, you need more water movement.
Filter throughput for a saltwater aquarium should be at least five times the total tank capacity per hour. For example, a 100-gallon tank needs 500 gallons per hour of turnover or more. Pumps capable of delivering such flow rates necessarily create water currents.
Caring for an Aquarium 25
Caring for an Aquarium 25
You must decide if your saltwater aquarium is to house any organisms that depend upon photosynthesis for their survival. This includes seaweeds and a host of invertebrate animals that harbor photosynthetic symbiotic partners. Make the critical decisions about lighting early in the design process. Aquarium lighting should show off the underwater scene to its best advantage and, if necessary, provide energy for photosynthesis. If the design relies solely on plastic reproductions or coral skeletons, then a single fluorescent lamp positioned over the tank may be enough. Even in an all-plastic ecosystem, more light always makes the tank appear inviting and fosters the growth of filamentous algae upon which many fish feed. Sometimes, unconventional lighting (by which I mean anything in addition to, or other than, the standard fluorescent strip across the top of the tank) can be used to produce striking effects. For example, a spotlight shining in can direct the eye toward a particular underwater feature, in much the same way that stage lighting directs the attention of the audience.
In the tropics, corals of all types reach their greatest abundance and diversity in clear, shallow waters, such as the shallows off the Florida Keys. Under such conditions, sunlight penetrates well. Even under the most favorable circumstances, however, the amount of available light underwater is only a fraction of that shining on the surface. Reflection, absorption with increasing depth, and turbidity all limit light availability in the reef environment. Even so, enough light for photosynthesis can reach the bottom to support dense growth because sunlight is quite intense. Few home aquariums rely on sunlight as the main light source and must make do with artificial lighting. Choosing an artificial-lighting system for a particular aquarium design requires knowledge of the available types of lighting equipment and their respective capabilities. In order to make comparisons, we must first define the terms used to describe light sources and light intensity.
The amount of light energy emanating from a source is measured in units known as lumens. The light intensity, or irradiance, over a given area is measured in lux, or lumens per square meter. Over a cornfield in Iowa in the middle of summer, the midday sun may provide irradiance of 100,000 lux or more. You'd be lucky to find an aquarium lighting system that can deliver 10 percent of this amount to the tank underneath it.
Several factors conspire to limit the efficiency of aquarium lighting. For example, the reflector housing the lamps cannot be 100-percent perfect, and therefore not all light emitted will reach the water surface. Reflection from the water surface itself reduces light penetration, too. Further, as the tank becomes taller, the amount of light reaching the bottom decreases dramatically due to the inverse square law of optics. Light intensity decreases in proportion to the square of the distance between the source and the object illuminated. In practical terms this means that the same light fixture over a tank twelve inches in height delivers only one-fourth as much light to the bottom if the height of the tank is increased to twenty-four inches. Double the distance, and illumination decreases fourfold. Further, the greater height of the water column means more absorption by water itself. This again reduces the effective light intensity.
The implications for aquarium lighting design are straightforward:
• For aquariums up to about twelve inches in height, two fluorescent lamps of the maximum length that can be accommodated across the length of the tank should be used.
• For deeper tanks up to four feet long, use four fluorescent lamps of the maximum possible length.
• For larger tanks, use one to several metal halide lamps to provide extremely bright light.
Although I suggest here choosing lamps by length, in actual practice it is the wattage that matters; the higher the wattage, the brighter the lamp. For example, a lamp four feet long consumes 40 watts of electricity and produces about 3000 lumens. Data on the lumen output of various types of lamps can be found on lighting manufacturer's Web sites. Appendix C, "Tank Specifications," provides lumen requirements for all the standard types of aquarium tanks.
For saltwater applications, several types of special lighting exist. For example, as one descends to greater depths, sunlight becomes selectively attenuated, with mostly blue wavelengths reaching the organisms. Many aquarists use actinic lighting to mimic these conditions. Where appropriate, I have included special lighting recommendations for some of the model designs given later.
Natural lighting varies as the sun first climbs and then descends across the sky. Cloud cover, reflection due to water movement at the surface, and turbidity, not to mention water depth, all affect the amount of light actually reaching marine organisms. If you obtain captive-propagated coral specimens, you may be able to learn the lighting conditions under which they were grown. Seldom do you have this information from a collected specimen. Therefore, some experimentation may be needed to optimally light any given item you obtain. As a rule of thumb, provide illumination that averages around 5000 lux over the course of a day. Thus, a forty-gallon long-style tank has 0.4 square meters and requires about 2000 lumens to achieve an irradiance of 5000 lux. You can check lumen output data for various lamps on the manufacturers' Web site. Use the average lumen value, if it is given. Reduce this number by 30 percent to allow for losses due to reflector inefficiency, reflection from the water surface, etc. Then total up the number of individual lamps you require to achieve the proper level of irradiance.
The length of the day is an important factor in regulating the growth of many species, and coral reef denizens are no exception. Reef fish and invertebrates usually do best with twelve hours of light daily. Use a timer to control the lighting system and provide a consistent day-night cycle. Large installations with complex lighting systems can mimic not only dawn-to-dusk fluctuations but also incorporate night illumination corresponding to the phases of the moon. While it is certainly not necessary to go to such lengths to have a successful reef tank, the lunar cycle definitely influences the reproductive cycle of many corals in their natural habitat.
Lighting a Living Reef Tank
You cannot grow corals and their relatives, or giant clams, or seaweeds without sufficient light, but unless the water conditions are also correct, you will end up growing only filamentous algae, even if you have the best lighting system on the market. Besides light, seaweeds and invertebrates need pollution-free seawater with the correct chemical and physical parameters. If you aspire to owning a living reef tank, you should be aware that invertebrates are less tolerant than fish.
Caring for an Aquarium 27
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