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What is considered a safe number of fish in an aquarium? What factors can determine or limit this number? This is a very common dilemma facing anyone who keeps fish - tropical or coolwater. From beginner to advanced aquarist there seems to be a universal urge to add "just a few more fish" to a tank, especially after all has been going great for weeks. Carrying capacity - the number of fish (or the weight of those fish) in the aquarium - is a topic well worth spending time with. The objective is to establish a framework from which we can better understand the bioload factors affecting fish selection and thus enhance the success of our fishkeeping efforts. Over the years, a few guidelines have been commonly cited, and these are now treated as "laws" to be passed down to new hobbyists. The most common of these is to stock a tank at "1 inch of fish per gallon of water" (a variation of which is "1 inch of fish per square foot of tank surface area").

In reality, these guidelines are simply not very useful. What is a standard fish inch? Does "per gallon" mean with gravel and rocks and plants...or without? Can a guppy be compared to an adult red devil? The single most critical factor in determining the carrying capacity of an aquarium is not the fish, not the filter, not the aquarium. It is the aquarist. The aquarist alone is responsible for feeding the fish, servicing and cleaning the filter, and changing the water. These are the critical processes that determine water quality...and thus, the carrying capacity of the aquarium. Consider this. An aquarium filtration system is not really about filtering so many gallons of water an hour, but rather about filtering the waste products of the organisms in the tank. The waste products are the result of the amount of feed given to the inhabitants. A filter that may be adequate for a 30-gallon tank with a fish population of 30 zebra danios can be wholly inadequate for the same tank if it contains 10 large fancy goldfish. Why? The 10 goldfish are producing significantly more waste than the 30 danios because they are eating so much more.

Let's examine the filtration of a heavily loaded aquarium in a couple of ways. Initially, let's say that the water in the aquarium is perfect for the fish and is completely replaced every hour with new fresh water. Aquaculturists consider this an open system. Water is used once, flows out of the tank and is replaced by new water. This environment is relatively stable as long as the incoming water does not change. There is also little ammonia buildup (it is flushed out of the tank). Because a biofilter is not necessary, nitrite and nitrate are not produced. The pH is stable and the oxygen needs of the fish are met by vigorous aeration. This type of tank can support a heavy load of fish, but it is not realistic for a home aquarium. Now consider the same tank in the other extreme. The unlimited water supply is eliminated. The water initially used to fill the tank is all there is. This is a closed system because no new water enters the tank and must be recycled through a filter. The filter's function is to remove particulate material and dissolved pollutants, and maintain low ammonia levels. If the filter were 100% effective, it would return the water to the state it was in when it first entered the tank. The fish load is similar to that in the first example. Filtration systems, as good as they are, are not 100% effective. Thus, the water does not re-enter the aquarium in pristine condition, as if never polluted. It is this inability to maintain perfect water quality that limits the fish load (bioload) that is safe for the tank. The first limiting factor in a fish system is not the presence of ammonia or nitrite, but oxygen. Fish must have oxygen to breathe. The more fish in the water, the more oxygen that must be supplied. Fish are not the only consumers of oxygen in an aquarium. Bacteria also consume a major portion of the oxygen. As fish numbers increase, there is more ammonia to oxidize, more waste to mineralize, and therefore more bacteria perform these tasks. More bacteria land increased bacterial activity require yet more oxygen. Fortunately, oxygen is relatively easy to provide in aquariums.

The most effective oxygen-generating methods create a lot of water surface agitation which is where gas exchange takes place. Surface agitation is important because it helps get rid of carbon dioxide...another fish-limiting factor. Carbon dioxide is more soluble in water than oxygen, so vigorous surface agitation is needed to drive the carbon dioxide from the water. As with humans, fish respiration produces carbon dioxide. Because it can be toxic to fish (and also lowers the pH), carbon dioxide must be controlled. It always astonishes hobbyists that a fish can die of carbon dioxide poisoning (suffocate) even in water that is saturated with oxygen. Some aquarists rely on plants to consume carbon dioxide and produce oxygen, but this will not work well in a heavily stocked tank. At night, when the plants are not photosynthesizing, they start to consume oxygen, creating an even greater demand for it. Ammonia is also a limiting factor. It is excreted by the fish in proportion to the amount of food they are given. It is also produced by bacteria when they degrade (convert) organic matter - a process called mineralization. High levels of ammonia are acutely toxic thus exposed fish will die in a very short time. Even constant low levels of ammonia are harmful because they limit fish growth. An old adage says that fish will only grow to the size of the aquarium. This is wrong - they will only grow to the size (and efficiency) of the filtration system. A university study on channel catfish showed that as little as 0.5 mg/L of un-ionized ammonia caused a 50-percent decrease in fish growth compared to fish grown in tanks with no ammonia. A concentration of nearly 1.0 mg/L resulted in no growth of the catfish at all. Ammonia is removed by nitrification: the bacterial oxidation of ammonia to form nitrite and then nitrate. But, nitrification also produces hydrogen ions, thus lowering the pH. As the pH drops, nitrification slows because nitrifying bacteria does not work as efficiently at lower pH values. This causes ammonia to increase. As the pH drops and the ammonia increases, the fish become stressed and more susceptible to disease. Thus, pH itself can be a limiting factor. In general, freshwater fish can tolerate a wider range of pH values than saltwater fish. For most fish however, a pH value near or below 6 starts to cause stress.

This situation with reduced pH leads to a malady that could be called "new fish death syndrome". It occurs when one tries to add a new fish to an old established tank that has not been well-maintained. The new fish dies after just a day or two. Why? Fish in the tank have slowly acclimated to the decreasing pH. The new fish, coming from water of a much higher (and more normal) pH, is dropped into the aged aquarium and must immediately cope with a pH value two to three levels lower then it is accustomed to. The fish goes into shock and dies. As explained above, the pH dropped because of nitrification. Because it is critical to maintaining a zero ammonia level, nitrification cannot be eliminated. The rate at which the pH drops depends upon the natural alkalinity (buffering) of the water and how much nitrification is taking place. Nitrification is determined by the feeding amount which is, in turn, determined by the number of fish. So you can see the problem. The chemistry and biology of a fish aquarium are irrevocably intertwined. Changing one factor means that, in effect, all the others will change as well.

An additional limiting factor is the buildup of dissolved organic material technically referred to as Dissolved Organic Carbon (DOC). This organically based material is generated via a host of biological activities in the aquarium, most of which are the result of microbial processes. Studies have shown that high levels of DOC inhibit nitrification. Activated carbon and foam fractionation are two ways to reduce DOC levels. Nitrite, nitrate and phosphate are not limiting factors in fish tanks. Nitrite is toxic but, generally it only occurs in high concentrations during the break in period of a new aquarium. Nitrate and phosphate are not toxic but may contribute to algae blooms and make the aquarium aesthetically unappealing. The continual adding of fish to an established aquarium initiates a pattern - more fish means more food, which means more ammonia, which means more nitrification, which means a rapid drop in pH. The pattern will occur whether there are a few fish or dozens of fish. Granted, it will take longer to complete in the tank with a few fish, but it will inevitably take place.

The best, easiest and most effective way to break this destructive pattern is through regular partial water changes. The filtration system does not prevent it from occurring. Filtration removes particulate material (mechanical filtration), reduces the DOC (chemical filtration) and controls ammonia (biological filtration). Water changes restore alkalinity to the aquarium water. They help maintain a stable pH. They remove DOC and prevent it from accumulating. They also remove nitrate and phosphate from the system. Which brings us back to my opening statement. The aquarist is indeed the ultimate limiting influence governing the number of fish in an aquarium. The more fish, the more water changes needed. The aquarist is an integral part of the filtration system. The greater the bioload, the more the filtration system will have to be serviced.

The hobbyist who sticks to an appropriate schedule of water changes and servicing for the bioload will be able to successfully keep and maintain aquariums at a larger carrying capacity than those who are unable or unwilling to perform the necessary water changes and servicing. Each hobbyist must determine for themselves the level of effort they are willing to expend on their fish tanks and adjust the fish population accordingly. The result will be healthier fish and greater enjoyment of the hobbyist.


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