Updated: Jun 12, 2022
In part 1 of this series, I discussed three types of freshwater aquariums defined by their general hardness, carbonate hardness, and pH. Most freshwater aquariums qualify as soft, hard, or neutral as defined by these parameters.
The remaining parameters discussed in this article are listed in the table below. They include everything from temperature and organic nutrients to salt and trace elements.
5.5 - 9.5
5.5 - 9.5
4 - 8 dGH
0 - 18 dGH
4 - 7 dGH
0 - 10 dGH
68 - 82 degrees F
45 - 82 degrees F
< 0.1 ppm
< 0.1 ppm
< 5 ppm
< 0.05 ppm
0.005 - 0.05 ppm
10 ppb (3)
0.5 - 1 ppm (4)
5 - 10 ppm
0 - 20 ppm
1 - 100 ppm (5)
0 - 0.1 ppt
0 - 0.5 ppt (1)
5 - 15 ppm
5 - 15 ppm (1)
< 5 ppm (2)
5 - 20 ppm (1)
Temperature in The Freshwater Aquarium
For most tropical tanks, a temperature between 74 and 80 degrees Fahrenheit is ideal. Some species like discus prefer temperature up to 82 degrees F, and others like goldfish prefer lower temperature around 68 degrees F.
Temperature seems like a set it and forget it water parameter, but many factors can affect the temperature. In addition, fluctuating temperatures can affect your tanks oxygen levels, and your fishes’ metabolism and breeding behavior.
As water temperature decreases, its ability to hold dissolved oxygen increases. Dissolved oxygen is responsible for fish respiration and providing oxygenated environments to beneficial bacteria.
While the comfort of your fish and tank inhabitants is paramount, allowing the temperature to rise too high, especially in a tank with low flow can be detrimental and even deadly by reducing oxygen to unbreathable (anoxic) levels.
If you're dealing with a power outage, or you are transporting fish a long distance, keeping the temperature at the lower end of the safe range is preferable to allow the maximum amount of dissolved oxygen.
As the temperature increases, the metabolic rate of your aquarium's inhabitants also increases. This almost linear relationship is a reliable indicator of how much metabolic energy your livestock are spending. If you are keeping your temperature higher for breeding, or disease treatment purposes, expect your fish to need more food as their metabolic rate increases.
Increased metabolism also means more waste. Be sure to test your nitrate and phosphate levels while the temperature is higher.
An interesting behavior in riverine fishes, sometimes observed in captivity, is their tendency to initiate courtship and breeding behaviors whenever the temperature is lowered by a few degrees for a short period of time.
In the wild, the wet and spring seasons bring frequent and cooler rainstorms that replenish river systems. This time of the year is an indicator to most fish that increased food supply and habitat are coming and they have evolved to breed during this period (Gorski, et.al. 2010). The temperature change can be mimicked in the home aquarium by turning down the heater, or using cooler water when performing water changes. With either method, make sure the temperature does not decrease more than 1 degree a day and 2 - 4 degrees total.
Ammonia, Nitrite, and Nitrate
These three compounds all contain the element nitrogen (NH3, NO2, NO3). They are the three main components of the nitrogen cycle as it pertains to the aquarium. I won’t go into detail about the nitrogen cycle here, but suffice it to say, ammonia is the most toxic, followed by nitrite, then nitrate. For a complete explanation of the nitrogen cycle, download the Aquanomicon on the Fish Tank Biologist Blog page.
Ammonia should be converted to nitrite almost instantly if you have established biological filtration. If your tank is new, or you’ve added fish recently, and your beneficial bacteria population is too small to compensate for the new rate of ammonia production, you may experience a spike in ammonia.
Ideally, free and total ammonia (ammonia/NH3, ammonium/NH4) should be at 0 ppm (parts per million). A spike may elevate ammonia to 0.25, 0.5 or even 1 ppm. Ammonia can be toxic to fish and inverts at any level, but should definitely be considered an emergency above 0.5 ppm.
Ammonia can be reduced by adding beneficial bacteria, performing water changes, or adding zeolites to your filter. Lowering your pH by 0.2 to 0.4 ppm won't reduce ammonia, but it will reduce its toxicity to your fish. I don't recommend this method however, because elevated ammonia is already stressful, changing the pH as well will only increase stress on your fish.
Nitrite should also be at 0 ppm, as the biological filtration should convert it to nitrate instantly if the beneficial bacteria population is sufficient. Nitrite is less toxic than ammonia, but should be considered an emergency above 1 - 2 ppm. Water changes and adding nitrifying bacteria will help reduce nitrite.
Finally, nitrate is the least toxic, and should be considered an emergency above 30 ppm, and a water change necessary at 10 ppm. Nitrate will not convert further in the tank unless you have plants or anaerobic bacteria.
In addition to a complete microfauna, plants and chemical filtration can do wonders to keep your nutrient levels low. Planted tanks can run 0 ppm nitrates and phosphates without water changes.
Another major nutrient found in aquariums, which can cause algae problems, and can be toxic to inverts at higher levels, is phosphate (PO4). Phosphorus (P) is a major component of fish food and waste. In the aquarium, it quickly converts to phosphate where it contributes to algae growth at levels as high as 0.05 ppm. At extremely high levels, like 1 ppm, phosphate can become toxic to most invertebrates.
Granular ferric oxide (GFO) and healthy freshwater plants can make quick work of any phosphate issues you may be having. You can place GFO into a filter bag and place it in your filter, or install a reactor if your filtration is large enough to accommodate extra equipment.
Most people don’t have to deal with copper in their aquariums. It usually enters through tap water, someone dropping metals in the tank, or on purpose with copper medications. The safe limit of copper for most fish is 0.39 ppm. Most plants and all invertebrates find this level of copper to be toxic. Inverts are especially susceptible to copper, even at extremely low, trace amounts.
If you find copper in your aquarium, multiple water changes will reduce the amount significantly. To remove the very last remaining copper ions, use activated carbon with humic acid drops from your local fish store to pull all the metals from your water.
Iron and Potassium
These two elements are important for plant growth in freshwater aquariums and are usually supplemented as fertilizers. Iron should remain at a constant 0.1 ppm and potassium at 5 - 10 ppm. This will provide a sufficient amount for plants to uptake and use.
In a non-planted tank, trace amounts of iron and potassium are acceptable. Elevated levels of iron can become toxic eventually, but can be handled by removing the source and performing water changes.
Most commonly in the form of chlorine and chloramines, chlorine is found in most tap water. If you are using Reverse Osmosis Deionized (RODI) water with a sufficient filter to remove all chlorine, you won’t have to worry about chlorine or chloramine. If you are using tap water, you’ll need to add chlorine removing solutions to your water before adding it to your aquarium.
The functional level of chlorine in your tank should always be 0 ppm, as chlorine and chloramine are extremely toxic to fish and invertebrates. After you have used a chlorine removing agent, let the water sit for several minutes to ensure all chlorine and chloramine compounds have been denatured and destroyed.
For the purposes of this article, salinity refers to the concentration of the various types of salts that naturally occur in freshwater. While sodium chloride (NaCl) is the most common type of salt, and is usually what most people refer to when discussing the subject of using salt in the freshwater aquarium, there is also potassium chloride, magnesium chloride, calcium chloride, potassium sulfate, aluminum sulfate, iron sulfate, and potassium iodide. These are also considered salts and are included in the less than 1% salt threshold that defines fresh vs. salt water.
In terms of commercially available aquarium salt (NaCl), it is usually used to aid osmotic pressure or osmoregulation. Fish tissues are exposed to the water they live in. Water travels easily in and out of the body. Osmotic pressure refers to the balance of salts in fish tissues vs in the water column. Whichever has higher salt concentration will absorb the most water to balance the salt concentration on both sides.
For example, imagine a wall that has water on both sides. The wall is permeable and allows water, but not salt, to pass through it. On one side of the wall, the water has 2 times the amount of salt as the other. The laws of physics dictate that water from the side with less salt will flow into the side with more salt until both sides' salt levels are balanced. This process is called osmosis and fish undergo it continuously. If your aquarium has no salt in it, water will flow into your fish tissues to lower their salt concentration. If your aquarium is too salty, water will flow from your fish tissues and dry them out.
But NaCl isn’t the only salt found in freshwater. To best mimic the environment your fish live in, determine if their natural water is relatively low or high in salts and then mix in a blend of natural salts. With African cichlids, use a mix of the salts listed above and keep the general hardness between 3 and 6 dGH for lake Victoria, 4 - 8 dGH for Lake Malawi, and 8- 14 dGH for lake Tanganyika. Fish from the Mekong river in Southeast Asia and the Amazon river in South America don’t need very much salt at all. Concentrations below 4 dGH or a salinity below .05% are ideal.
As I mentioned earlier, dissolved oxygen (DO) levels increase as temperature decreases. DO also increases as water movement increases. The best way to make sure your water is more oxygenated is to agitate the surface of the water with good flow. This is where gas exchange between the atmosphere and the water column occurs. This is also why air bubblers are popular. They don’t provide oxygen directly via their bubbles as they are much too large to dissolve in the water. Instead, the bubbles agitate the surface of the water, promoting gas exchange, releasing oxygen from the atmosphere into the aquarium water. This same effect can be achieved with a powerhead, or by pointing your main filter return toward the water surface.
Photosynthetic plants and algae will also release oxygen into the water, but not with the efficiency of properly flowing water and sufficient surface agitation.
The only reason to supplement carbon dioxide into your tank is for plant growth. Fish and bacteria will respire, adding CO2 into the water column, but not enough for plants to use. Some CO2 will also mix into the water column with surface agitation, but again, not enough to significantly affect plant growth.
Supplementing CO2 with a gas cylinder, regulator, and diffuser will greatly increase plant growth and health. You can gauge the amount of CO2 to add by monitoring plant growth, utilizing a CO2 indicator solution, or by measuring pH before the CO2 is turned on in the morning, and measuring it again just before it is turned off. This last method works because CO2 converts bicarbonates into carbonates, which lowers the pH, and will give you an idea of how much CO2 is being added. You don’t want your pH to fluctuate from CO2 dosing more than 0.2 to 0.4 ppm in a 12 hour period. After the CO2 is turned off at night, pH will increase until the CO2 is turned back on again.
Of course, If you don’t have plants, or if you are running a low-tech setup (no CO2 dosing), then you need not concern yourself with the concentration of CO2 in your tank, as surface agitation will regulate it for you.
Creating the perfect environment for your fish through proper water parameters can be difficult and perfection will never be attainable. The key is to know what the natural value for each parameter is for the inhabitants of your tank, then try to mimic that value the best you can.
If you have fish from all over the world, and their parameters are different, you can adjust the values within the safe ranges until you find the number that benefits all your livestock. Just make sure you make adjustments slowly. Most fish can adjust to parameters slightly off from their natural values, but very few can survive a quick and drastic change.
Invertebrates are less forgiving. If you have certain types of snails or shrimp, always favor their ideal water parameters over the fish. Remember how we talked about osmosis and how fish tissues are exposed to their environment, well inverts even more so. Most invertebrates won’t tolerate even small fluctuations or improper water parameters even for a limited amount of time.
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Department of water, western australia
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Agency for Toxic Substances and Disease Registry Division of Toxicology CDC
Gorski, K., Winter, H.V., De Leeuw, J.J., Minin, A.E., Nagelkerke, L.A.J. "Fish spawning in a large temperate floodplain: the role of flooding and temperature." 2010. Freshwater Biology. 55:7, 1509-1519.