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Robyn's Water Chemistry Page

Last Updated: 7/14/07

Animated Bubbles

The Nitrogen Cycle - The cycle, changing water during the cycle, bacteria, cycling a new tank and fish choice, and what to do if the ammonia or nitrite spikes too high
Test Kits
Chemicals of Interest in Tanks and Ponds
Useful Conversions for Aquarists and Ponders
Dealing with Poisonings

I need to update, divide, and expand this page but in the mean time:
New pages that I wrote!
Water - types of water, chlorination, fluorination, and compressed gases
Water Comparison Table - attributes of the various types of water you might use

For information on foam in ponds due to DOC, see the pond care page.

I have a Masters degree in analytical chemistry so I know a lot about water chemistry. Feel free to ask me to explain any of the following in simpler terms or to expand on it.

See my fishy care page for information on water changes and water additives. If you have city water, remember that you must deal with the chlorine or chloramine in the water. For chlorine, add dechlorinator or let the water sit (better aerated) for a few days or a week before using it. If your water has chloramine (your city should give you a water report about once a year or contact them), then it will not dissipate and a dechlorinator and ammonia- neutralizer will have to be added (or treated by other less-common means). I am still amazed when I realize that someone is not aware of this and has been keeping fish for years (that continually were sick and often died from the chlorine burning their gills). One relative did not have this problem because they just never changed the water!

The Nitrogen Cycle

The Cycle:

A newly setup aquarium or pond does not contain very many bacteria. Once fish are introduced, ammonia (NH3) will begin to be added to the tank from uneaten food and fish wastes. In acidic waters (pH < 7), the ammonia will combine with hydrogen cations (H+) to create ammonium (NH4+) which is not very toxic. If the levels of ammonia become too high, they will burn the fishes' gills and make it hard for them to breathe. The fish will zip all around and die. Luckily, there is a naturally occurring bacteria in natural waters that converts ammonia into nitrite (NO2-). Over a few days or weeks, these bacteria will begin to grow in a new aquarium or pond. Then, the levels of ammonia go down while those of nitrite go up. Unfortunately, nitrite is even more poisonous to fish and kills them more easily. Again, luckily, there is a second bacteria that converts nitrite to nitrate (NO3-). This takes a few more weeks. Nitrate is only harmful in very high concentrations. Live plants use ammonia and nitrate as food. Water changes reduce the concentrations of ammonia, nitrite, and nitrate.

Changing Water During the Cycle:

Much has been said about the nitrogen cycle and its importance. For people who change very little of their water and not often, this is very important. Also, for ponds, it is important initially. But, if you have just a few fish, add bacteria, and change 30-50% of the water every week with treated similar water, then the nitrogen cycle is not so important (ammonia and nitrite will never be readable). Some aquarists say not to change water as the cycle gets set up but I believe it is better for the cycle to take a bit longer with water changes than to kill or almost kill the fish in a rush to get the cycle "completed." Changing water during the cycle will only delay its establishment, it will not halt the process.


I also add Stress-Zyme which has live bacteria in it to help the nitrogen cycle. There are many such products (Cycle is another) and conflicting reports on whether they do any good at all. In water above pH 7, two groups or species of bacteria (controversy now abounds on what species actually do it) convert ammonia to nitrite to nitrate (NH3 =>NO2- => NO3-). In acidic water, it is ammonium (NH4+) which starts the process, which is less toxic than ammonia. Nitrite and ammonia can kill fish easily. Nitrate is less toxic and provides food for many live plants and algae. The helpful bacteria take a month or two to get up to full speed with the fish waste load. A pond may take a year to grow a healthy colony of bacteria. The bacteria do best between 70 and 90 degrees F. They die in too cold or hot water. In cold water, fish usually hibernate and produce less waste; therefore, the bacteria are not needed in winter. The key to keeping fish safe from their own wastes and other sources of ammonia and nitrite is to make water changes with vacuuming and to add only a few fish at a time when setting up a tank.

Cycling a New Tank and Fish Choice:

A tank can be cycled using fish (actually their wastes, other aquatic animals work as well) or ammonia added directly. Cycling a tank with fish means adding fish so that they produce ammonia to promote the growth of the bacteria that convert it to nitrite and then nitrate. A tank is "cycled" when ammonia and nitrite readings are sustainably zero (they stay zero for at least a few weeks), and nitrate readings increase. Many people tell new aquarists to cycle their tanks with cheap or trash fish like feeder fish, danios, guppies, etc., and then to get rid of them or return them to the store. I do not believe this to be a good choice. First, these fish are not respected and being treated poorly are most likely full of diseases, bacteria, and/or parasites. These are then introduced into the new tank and stay there. Second, after the tank is cycled, the fish have to go somewhere. If unwanted fish were used, they either go back to the store (in which case the stress most likely kills them) or are killed outright. It is much better to use fish that one intends to keep to cycle the tank. There is NO reason that a single initially-healthy fish should die during cycling (see information above on water changes). Anyone who tells you that it is "normal" for a fish to die is misinformed or trying to sell you more fish.

The fish desired in the tank should be determined ahead of time. A few of the most hardy of those should be the first inhabitants of the tank that will get the nitrogen cycle going. These fish will remain in the tank when the tank is cycled, and new fish are added. Good cycling fish that can be kept in most community tanks include fancy guppies, small danios, small tetras, small barbs, white cloud mountain minnows, etc. Even most so-called sensitive fish can be used to cycle a tank without dying. In rare cases, it may actually be necessary to cycle a tank with fish that cannot remain in the tank forever (like a piranha tank). Ideally, these fish would be moved to a tank of their own after cycling. Algae eating and bottom dwelling fish such as plecostomus, otocinclus, algae eaters, snails, corydoras catfish (cories), and freshwater shrimp should not be added until after the tank has been set up for a while and algae and bottom debris are present.

What to Do if the Ammonia or Nitrite Spikes too High:

Often, in small tanks (10 gallons or less) or overstocked tanks, the nitrogen cycle can be deadly. This occurs when the ammonia (and later nitrite) produced by the fish, food, etc. is simply so high that it begins to harm and kill fish. Once bacteria become established, the levels will go down to zero but what to do in the mean time? First, if the levels of ammonia or nitrite ever read in the test kit's danger zones, doing nothing will most likely kill the fish. The most important thing to do initially is to get the levels down to acceptable ranges. For tanks under 50 gallons, doing a 50% water change (less water in larger tanks) with gravel vacuuming EVERY day until the levels are below the danger zone (but still present) will help ease the stress on the fish and yet allow the good bacteria to develop. Yes, it will take longer but then the fish might actually survive. The addition of liquid bacteria like Stress-Zyme or Cycle or dry bacteria in larger ponds especially, will help get the good bacteria growing. Because ammonia (NH3) is in the form of ammonium (NH4+) when the pH is acidic, lowering the pH if the aquarium water is basic (above 7) will render the ammonia less toxic to the fish. The pH may be lowered using commercial solutions or with vinegar (acetic acid) but should be done slowly and carefully. Liquid Ammo-Lock by Aquarium Pharmaceuticals can be added to bind some ammonia if present. Zeolite (a white granular compound, that looks like white rocks, often sold mixed with carbon) can be added to the filter (in a mesh bag or pantyhose or media container) or even put right in the tank or pond (in a bag of some sort) to absorb ammonia as well. Adding large doses of salt at least a tablespoon per 5 gallons will help detoxify nitrite if that is the problem. Keep in mind that salt is harmful to plants and some sensitive fish in high doses. Once nitrate test kits register nitrate present, the danger has most likely passed. This takes about a month or so.

Also, note that salt should be used sparingly if zeolite is also being used. This is because zeolite can be refreshed using sodium salts which replace the ammonia in the zeolite. If lots of salt is added to a tank or pond with zeolite that is full of ammonia, the zeolite may release noticeable amounts of ammonia which could be harmful. Use the zeolite to remove ammonia and later, if nitrite becomes too high, use salt to render it less toxic and remove the zeolite. Using small amounts of salt in a tank or pond with zeolite should not make much difference.

Web Sites:

A site with a series of interesting articles on water chemistry, filtration, bacteria, and more can be found at Dr. Tim's articles and Dr. Tim's reports.

Test Kits

Test kits are available for pH, ammonia, nitrite, nitrate, oxygen, carbon dioxide, phosphate, iron, hardness, alkalinity, chlorine, and more. The most important test kits to have are those for pH and ammonia. The next important are nitrite, nitrate, and hardness. If you have city water, you should have a test kit for chlorine and/or chloramine. The other kits are more important for people with planted or marine tanks. Test kits may no longer work properly past their expiration dates so check those before using a kit. Most kits register a color change for comparison against a chart. More expensive kits tend to be more accurate but this may not be too important if you want to know simply if the compound (such as ammonia) is present. A tank or pond that is "cycled" should never register any amount of ammonia or nitrite.

Chemicals of Interest in Tanks and Ponds

Under Construction (so there are blanks; I hope to get to it some day if I have time to look it up!)!

Water Chemistry

Chemical Definitions and Conversions Acceptable Ranges Method of Alteration
pH or -log[H3O+] The acidity or concentration of hydronium cations in the water. pH <7 is acidic, pH >7 is basic. Down to pH 5 for acidic fish and up to pH 9 for basic-loving fish, pH 7 is neutral and best for the majority of fish HCl (hydrochloric acid) or H2SO4 (sulfuric acid; pH Down) to lower pH; Na2HCO3 (sodium bicarbonate) to increase pH; add only small amounts at any time and adjust pH slowly.
Ammonia or NH3 Below pH 7, the predominate form is the less toxic ammonium cation (NH4+) Zero Water changes
Nitrite or NO2-   Zero Water changes
Nitrate or NO3-     Water changes
Chlorine (Cl-) or chloramine (Cl-, NH3 combination) Added to city water to kill all life   De-chlorinators should be added to city water to neutralize chlorine and/or chloramine; aeration will drive off chlorine but not chloramine
Hardness Total calcium (Ca2+) and magnesium (Mg2+) cations as well as other cations to a lesser extent; expressed as ppm (parts per million, milligrams per liter), GH/KH measurements [total or general hardness and temporary or carbonate hardness (which can be removed with boiling, close to alkalinity) respectively], or dH (degrees hardness); 1 dH = 17.9 ppm as CaCO3 Varies with species being kept; 0-50 ppm = soft; 50-100 ppm = moderately soft; 100-200 = slightly hard; 200-300 = moderately hard; >300 = very hard; I read 100 ppm is best for ponds with a range of 80-150 being okay Using calcium rich gravels and shells can increase hardness or calcium salts can be added directly to the water; add baking soda to increase KH in ponds [the ounces needed = (desired ppm increase) x (pond volume in gallons) / 5000]; dilute with softer water to decrease hardness (unpolluted rain water or reverse osmosis or ion-exchanged water are often used)
Alkalinity The capacity of the water to maintain its pH or the buffering capacity, results from the carbonates (HCO3-) and similar chemicals in the water   Generally increased using baking soda (sodium bicarbonate, NaHCO3)
Oxygen or O2(g)     Aeration will add oxygen to water low in oxygen and also drive off excess oxygen
Carbon dioxide of CO2(g)     Yeast reactors or compressed carbon dioxide can be used to add carbon dioxide to planted tanks, aeration will add carbon dioxide to water low in carbon dioxide and also drive off excess carbon dioxide
Iron or Fe     Liquid plant food with iron, laterite, Flourite (brand name)
Phosphate or HPO4-2   Low Water changes and/or resins to reduce, fertilizers to increase
Salt or NaCl   Varies with species kept; I read 0.1% is best for fish ponds Water changes will reduce the salt content while adding salt increases the content

Useful Conversions for Aquarists and Ponders


g = grams, lb = pounds, oz = ounces (apoth or troy)

L = liters, mL = milliliters, gal = gallons in US, pt = pints, qt = quarts, tsp = teaspoons, Tbsp = tablespoons, fl oz = fluid ounces, ft3 = cubic feet

deg F = degrees in Fahrenheit, deg C = degrees in Celcius


For more conversions, see our Calculator / Converter or the Online Conversion web site.



Weights to Volumes:


Example for most common temperature for keeping most tropical fish at 75 deg F:
deg C = (5/9)(75-32) = 23.9 deg C
Therefore, 75 degrees F = 23.9 degrees C.


I want my pond to have 0.1% salt concentration in it. The pond is 5 feet wide by 10 feet long by 3 feet deep. How much salt do I add?

A 0.1% solution means that for every 100 grams of water, there should be 0.1 grams of salt. Since 1 g water takes up 1 mL, that means 0.1 g salt per 100 mL water or per 0.1 L of water. Since 1 gallon of water is 3.785 L, then 1 L is 0.2642 gallons. Thus, you need 0.1 g of salt per 0.0264 gallons or 1 g of salt per 0.264 gallons. Since 1 cubic foot of water is 7.479 gallons, then 0.264 gallons is 0.0353 cubic feet. Thus, you need 0.1 g of salt per 0.0353 cubic feet which is the same as 28.3 g per cubic foot. Your pond is 5'x10'x3' or 150 cubic feet. Thus, you need 28.3x150 or 4250 g of salt for your pond. Since one pound of salt is 497.7 grams, then this is equal to 8.5 pounds of salt.

Note that you cannot convert this into cups of salt to add because it depends on whether the salt is powder, little crystals, or big crystals as to how much room it takes up. In many cases, the container of salt you use has both a volume and a weight on it. For example, if the container is 10 pounds in 1 gallon, then you need 0.85 gallons or 13.6 cups (0.85*16) which is also 13 cups and almost 10 Tablespoons. By the way, your pond is 1121.85 gallons (150 x 7.479). Mathematically, this is all shown below.

0.1 g salt/100 g water x 1 g water/1 mL water x 1000 mL/ 1 L x 3.785 L/1 gal x

7.479 gal/1 ft3 x 5 ft x 10 ft x 3 ft x 1 lb/497.7 g = 8.5 lb of salt

I don't quite understand the calculations above. Can you make it easier for me to understand in summary?:
In order to get a 0.1% solution of salt in a salt-less pond, you need to add 0.75 pounds of salt per 100 gallons of pond water.

Dealing with Poisonings

An aquarium or pond may become poisoned by any number of chemicals by accident or on purpose by nasty or careless people. When this happens, we hope that few animals die. Here are some ideas for what to do if paint, oil, gasoline, other chemicals, etc. end up in the water in amounts great enough to cause stress or death of aquatic animals.

1. Do an emergency water change first, at least 50%. If the tank or aquarium is small enough, aerate some fresh treated tap or well water and net the animals into a temporary holding tank right away. Then, do a 100% water change. If the situation is bad, the filter material may have to be tossed and the ornaments, etc. all cleaned as well. Remove any dead animals.

2. Put fresh carbon in the filter, lots of it. This will remove a lot of chemicals. Toss the carbon after a day or two depending on how bad the poisoning was. Replace with fresh carbon again. Repeat as needed. Keep spare carbon on hand for this.

3. Good bacteria may be killed by the poison or the attempts to clean the tank or pond. Add some good bacteria, both to help with biological filtration and to perhaps digest any remaining organic chemicals if they were the problem. For biological filtration, something like Stress-Zyme would help. For general good bacteria (not biological filtration but digestion of organics), I use BZT but the easiest one to find is Microbe-Lift for ponds.

4. Increase aeration. An air stone on an air pump, another aerator, or even a pump moving the surface will help. When under stress, having extra oxygen helps the fish. Keep this going for a few weeks or until things are okay.

5. Add some Stress-Coat, not really for dechlorination but to add aloe which will help them heal from the stress of the poison, water changes, and moves if any.

6. If the fish or animals show signs of stress, do additional partial water changes.

8. Monitor the tank or pond's pH, ammonia, and nitrite for a few weeks. The ammonia and nitrite might spike since some good bacteria were killed.

9. Add some aquarium salt to reduce stress. See the box for directions.

10. Rags and paper towels may help soak up oil or gasoline from the surface of water.

11. For ponds, in general, except in emergencies, such as with the addition of oil or paint, it is NOT a good idea to scrub down the liner as that removes good bacteria and micro-life helping to run the pond.

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