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Water quality - the chemical and physical characteristics of your aquarium water - is the single most important factor determining the success of your aquarium. Success means not merely keeping your fish alive, but keeping them healthy so they live their full lifespan and display their natural behaviors. Water chemistry is often the key factor leading to (or blocking) spawning in the aquarium. Sure, most freshwater tropical fish can survive in an astonishing range of water conditions, but being alive doesn't necessarily equate with being healthy. You could live for many weeks without food, but would you be healthy, happy, and behaving normally? The answer is no.
We've established that aquarium water quality needs to be "good" to promote the health and longenvity of our aquatic pets, but what constitutes "good" water quality? Unfortunately, there is no single set of parameters that makes water "good". The proper water chemistry for your aquarium depends on what kind of fish you are trying to keep. For example, South American Discus can live reasonably well in hard, alkaline water, but chances are that they will never reproduce because their natural habitat is soft, acidic water. To be truly healthy and live their full lives, fish actually require quite narrow ranges of most most water quality parameters.
Water quality is dynamic. Once you achieve good water quality, the challenge becomes maintaining it. Simply having fish in the system and feeding them starts chemical and biological processes that degrade water quality over time. These processes include the production of acid by the nitrifying bacteria in the gravel and filter (not to mention respiring fish and plants), which means that over time the pH of aquarium water will always tend to drop. Others are the constant formation of nitrate by these same bacteria and inputs of dissolved organic compounds from fish waste and uneaten food. The way to combat these changes and keep the water chemistry stable is to have an efficient, well-maintained filter system.
Because water quality is dynamic you need to test it regularly if you hope to maintain a stable environment for your fish. Testing your water is critical because it is the only window you have into the quality of your water. Well, the fish would eventually signal poor water quality by getting sick or dying, but this is what we're trying to avoid! Water testing is important, and you need to do it, but don't fall into the trap of chasing numbers. Some aquarists read that the pH should be X and hardness needs to be Y, and continually add chemicals and buffers to the water to alter the pH and hardness, resulting in wild swings in water chemistry. AVOID THIS! While most fish can tolerate a range of water chemistry, they DO NOT tolerate rapid changes. Maintaining stable water chemistry, and keeping it at the optimal levels for your particular fish, is difficult. Many of the parameters are interdependent, so that if you change one, the other changes as well. Being able to manage and manipulate water chemistry to provide a stable, healthy environment is the real challenge facing any aquarist. This section will attempt to give you a working knowledge of the important aspects of aquarium water chemistry and will hopefully put you on the road toward better water quality. |
| The Nitrogen Cycle (Ammonia, Nitrite, and Nitrate) |
Three of the most important water chemistry parameters are ammonia/ ammonium (NH3/NH4+), nitrite (NO2-), and nitrate (NO3-). These three chemicals are important because ammonia and nitrite are both very toxic to aquatic life, even at low concentrations. Incidentally, neither one is great for us either. Nitrate, while much less toxic than either ammonia or nitrite, is toxic at high concentrations. More importantly, it is a great plant nutrient and thus causes algal blooms. Most cases of green water are due to having huge amounts of nitrate (and phosphate) in the system. The best way to manage these potentially dangerous chemicals is to understand the nitrogen cycle, since this is the best tool for ammonia and nitrite removal.
There are a variety of products out there that claim to detoxify ammonia and nitrite. These preparations are acceptable to use in case of an emergency, like when a dead fish causes a huge ammonia spike, but they are not a substitute for good filtration and husbandry. These things do not actually remove any nitrogenous waste from the system, they simply bind to ammonia and render it less toxic to fish. All the nitrogen is still in the system feeding other bacterial and algal populations. Sorry, but there's no magic elixer that instantly gives great water quality. There's still no substitute for biological filtration and good, old-fashioned water changes.
Now, back to the nitrogen cycle. All nutrient inputs into the system, primarily carbon and nitrogen compounds, stay in the system unless they are removed by water changes. Carbon will be discussed later, and we will focus on nitrogen for now. As the diagram below shows, some portion of all nutrient inputs (the proteins, carbohydrates, and fats in fish food primarily) ultimately end up as ammonia. Whether the food is eaten by fish and expelled as urine and excrement, or simply decomposed in the gravel by nitrifying bacteria, ammonia is the end result. So without an active biological filter, food inputs will lead to a rapid accumulation of ammonia. |
Food put into the system will go directly to ammonia as fish waste, or if left uneaten, will be decomposed to ammonia. The ammonia will then be broken down by ammonia oxidizing bacteria (Nitrosomonas) to nitrite. Nitrite is further oxidized by nitrite oxidizing bacteria (Nitrobacter/Nitrospira) to nitrate. Both ammonia and nitrate are taken up by plants. This means that plants are an excellent means of controlling both ammonia and nitrate. Unfortunately, nitrate also feeds algae. Plant and algal debris falls to the bottom of the tank where it is decomposed to ammonia and the dance continues... Water changes remove all of these things from the water, and dilute whatever remains. |
| Luckily, several species of bacteria come the aquarist's rescue. The first group of bacteria, belonging to the genus Nitrosomonas, get most of their energy by converting ammonia to nitrite. These are the ammonia oxidizing bacteria (AOB). Good news for aquarists! Even better is the fact that these guys will show up no matter what. Unless you put bleach or antibiotic in the system, you will eventually get a healthy population of Nitrosomonas. At this point, your faithful horde of Nitrosomonas are busily converting ammonia to nitrite. Unfortunately, nitrite is still toxic to fish. Again, becteria come to the rescue. This time Nitrobacter and/or Nitrospira show up to consume the nitrite and convert it to nitrate. These are the nitrite oxidizing bacteria (NOB). Nitrate is much less toxic than either ammonia or nitrite. Basically, nitrate will only be a problem if either it begins to fuel rampant algal blooms, or it accumulates to a tremendous concentration (> 50ppm). It is possible to grow bacteria to convert nitrate to gaseous nitrogen (N2), but this is difficult since these bacteria require anaerobic (no oxygen) conditions. This is a very rough sketch of the nitrogen cycle as it occurs in your aquarium. Food and detritus are converted to ammonia (among other things) by the fish and decomposing bacteria in the tank. The ammonia is transformed into nitrite by Nitrosomonas and the nitrite into nitrate by Nitrobacter and/or Nitrospira. Together, these two processes are called nitrification, and the bacteria that perform these biochemical conversions are called nitrifying bacteria. Denitrification refers to the anaerobic conversion of ammonia to gaseous nitrogen, but this does not occur in most freshwater aquariums. Both ammonia and nitrate are used as food by plants, thus the nitrogenous waste is converted into plant tissue. When the plants die, they decompose ultimately into ammonia and the cycle continues. |
| Cycling an Aquarium |
Probably the most important step in starting a new aquarium is letting the tank cycle. Cycling is the process that starts the biological filter. During the cycle, the necessary colonies of Nitrosomonas and Nitrospira each take hold and grow. The colonization of the filter and gravel by Nitrosomonas (AOB) initiates the nitrogen cycle. After this, a natural succession occurs where the AOB convert ammonia in the aquarium to nitrite. Before the next wave of bacteria, Nitrobacter and/or Nitrospira, can colonize the aquarium two conditions must be met. First, there must be sufficient nitrite in the water for these NOB to take hold. Second, since growth of these bacteria is somewhat retarded by ammonia, the ammonia concentration must be brought nearly to zero by the AOB. So there is a population dynamic where the AOB population explodes because of the large amounts of ammonia in the water. They quickly use this up and produce a nitrite spike. Since the ammonia level is low, the AOB population starts to die back. At the same time, the nitrite spike fuels an explosion of NOB. When the nitrite is used up, the NOB start to die back. All of these dying bacteria, together with whatever food is being added to the system, decomposes to ammonia, which feeds the AOB population. The pendulum swings back and forth until the AOB population just keeps up with ammonia input, and the NOB population just keeps up with nitrite production. When these cultures are in equilibrium, the ammonia and nitrite concentrations are maintained at or near zero and the tank is said to be cycled.
The procedure for cycling a tank is simple. Just fill the tank with water, turn on the filter, and wait. Establishing the necessary bacterial cultures is not a rapid process and can take four to six weeks. Be patient! The only things you should do to hurry the cycle along is add small amounts of food, or add some gravel or filter media from a healthy, established aquarium (or both). Flake food or tiny bits of shrimp work well. These small food inputs will provide ammonia and eventually nitrite to feed the cultures. Many tropical fish retailers sell cultures of nitrifying bacteria to make the tank cycle more quickly by establishing the bacterial populations faster. The number of living cells in these preparations varies greatly, and I have not read of anyone strongly recommending them. Many older aquarium books and most fish retailers tell people to cycle the tank using one or two hardy fish. This is not only unnecessary, it's cruel. The water in a brand new aquarium is nowhere near adequate for supporting aquatic life! Test the water for ammonia and nitrite once a week for the first two weeks, then every few days after that. Once the levels reach zero, it is safe to begin stocking the aquarium - slowly. Add fish one or two at a time if they are large, 3-6 is O.K. if they are small. If too many fish are added too quickly the waste they produce can overwhelm the biological filter and cause an ammonia spike. Remember that any sort of medication added to the main tank will kill your biological filter as well as the disease you are treating. This effectively "uncycles" the tank. If you medicate you main tank (I advise against it - get a cheap hospital tank) you need to pay careful attention to ammonia and nitrite levels and be ready to do daily water changes. |
This diagram shows the progression of ammonia, nitrite, and nitrate levels during cycling. The values were taken from my African cichlid tank when I first set it up. I colored the curves to illustrate the toxicity of the various compounds. While there is a small green "safe" zone marked for ammonia and nitrite, these should be maintained at zero in the aquarium. I'm sure there is a "safe" level of carbon monoxide, I wouldn't buy a house that had it! |
| pH, Alkalinity, Hardness, and Carbon Dioxide |
| Hardness, alkalinity, pH, and carbon dioxide (CO2) concentration are all interdependent variables. pH is the measure of how acidic (or basic) the water is. More specifically, it is the concentration of hydrogen ions (H+) in the water. High hydrogen ion concentration means the pH is low (acidic) and a low H+ concentration means the pH is high (basic). The pH scale runs from zero to fourteen with zero being the most acidic and 7 being neutral. Vinegar, for example, is pH 3, baking soda solution about 8.4, and the pH of natural stream water can range from 6 to 8 depending on the geology of the stream bed. Alkalinity is a confusing term because different authors use it to mean different things. Alkaline is often used as the opposite of acidic. I always use basic as the opposite of acidic to avoid confusion, since alkalinity is something different (but related). Alkalinity refers to the buffering capacity of water. A buffer is a chemical that allows a solution to resist changes in pH. Buffers are often weak acids that partially ionize in solution, CH3COOH, acetic acid. In solution acetic acid ionizes to form CH3COO-, it's conjugate base. The base form of acetic acid, acetate, will sop up H+ from solution. This is one definition of a base - a compound that will accept protons (H+). If the acetate concentration is high enough, any extra H+ (acid) added to the water will be soaked up by the acetate. This keeps the H+ ion concentration, and so the pH, constant. In aquariums, the major buffers are almost always carbonates and bicarbonates. So, alkalinity in aquariums is measured by the concentration of carbonates in the water. This is also referred to as "carbonate hardness" or kH. General hardness (also known as permanent hardness or gH) is the measure of all of the minerals dissolved in the water, mainly calcium and magnesium salts. Hard water has high concentrations of calcium and magnesium, while the opposite is true of soft water. The concentration of ions is important for the transport of nutrients across cell membanes and can affect reproduction and egg fertility. When fish are said to prefer hard or soft water, it is gH that is being refered to. kH does not directly impact fish physiology. Carbon dioxide in the water comes from respiring fish, plants, and bacteria. It forms carbonic acid in solution lowering the pH of the water as it accumulates. This is why some alkalinity (carbonate hardness) is necessary - it helps the water resist changes in pH due to CO2 accumulation. |
The pH, CO2, and dissolved oxygen levels all fluctuate during the course of the day. During daylight hours photosynthesis by aquarium plants and algae consume CO2 and release oxygen. This drives the pH up while increasing the concentration of dissolved oxygen. Since photosynthesis can not occur without light, plants respire at night. Respiration consumes dissolved oxygen and releases CO2. Thus the dissolved oxygen concentration falls while the CO2 contration rises. The carbon dioxide forms carbonic acid in solution and drives the pH down. |
| Dissolved Oxygen |
| Obviously, fish need oxygen to breath. But so do bacteria, and even plants, at night, require oxygen. Well oxygenated water is the halmark of a healthy aquarium. Since warm water, like that in all tropical fish aquariums, holds less oxygen than cold water, it is a challenge to keep the dissolved oxygen concentration high. Oxygen gets into the water at the suface, where the water is in contact with the air. The best way to promote gas exchange is to provide some gentle agitation of the suface, either with the filter outflow, a small water pump. Airstones also help, but they are not as efficient as surface agitation. It also helps to keep the oxygen demand low. Overstocked aquariums will obviously have a high oxygen demand, but there are larger sources of oxygen demand that are often not well known. Aquariums with an overabundance of uneaten food and rotting debris always have high oxygen demand (and low dissolved oxygen concentration) due to the huge populations of decomposers. Feeding lightly and understocking will go a long way toward maintaining a high dissolved oxygen concentration. Signs of oxygen stress in aquarium fish include rapid gill movements, gasping, and hanging out near the water surface where the oxygen concentration is higher. If you see fish displaying any of these behaviors do a water change immediately and provide aggressive surface agitation.
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| Temperature |
| Tropical fish thrive in a very narrow temperature range, most often 72-78F. Temperatures even slightly above or below this range will stress fish over time and could lead to disease. Rapid changes in temperature should definitely be avoided. Few things kill fish faster than a rapid temperature change. If you come home to find your aquarium overheating, don't do a 50% water change and bring the temperature down 20 degrees in 5 minutes. This will kill all but the hardiest fish. Have a good quality heater (preferably two) with a built-in thermostat to control the temperature. A thermometer is another absolutely required item. |
| Dissolved Organics |
| Dissolved organics are basically everything that decays in the tank and doesn't get turned into ammonia. Things like phenolics, terpenes, tannins, and other carbon compounds can dissolve in the water. These are the cause of smelly, turbid water. The only way to remove dissolved organics in a freshwater system is with activated carbon (change at least once a month, preferably every two weeks) and regular water changes. |
| Chlorine/Chloramines |
| Chlorine, or the more chemically stable chloramines (chlorine chemically bonded to ammonia), are added to municipal water supplies in order to sterilize them for human consumption. This is generally good for us, but potentially disasterous for your fish. chlorine irritates the skin and attacks the gill membranes, effectively suffocating fish. Chlorine poisoning looks similar to oxygen stress, with fish gasping and floating about at the water surface. Letting water sit for 24 hours should get rid of any chlorine present. Since chloramines are more stable, it takes about a week of sitting for them to leave. You can also boil water for five to ten minutes, but this is a hassle for larger tanks. If you do boil your water, let it cool (obviously) and agitate or aerate it prior to use to replace the oxygen driven off while boiling. |
| Phosphorus |
| Phosphorus is not a major issue for fish, and it is not normally necessary to test phosphate levels. However, if you are having problems with algal blooms (green, cloudy water) then you should check the phosphate level in the aquarium water. Phosphate is often the growth-limiting nutrient for plants in aquaria, so if your water has lots of phosphate, chances are you'll have lots of algae. The best ways to combat high phosphate and algae blooms is to cultivate some fast-growing plants that will hopefully out-compete the algae for phosphate. The other alternative is to buy a water purification system (reverse osmosis/dionization or RODI) to remove the phosphate. |
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