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Last Updated: 8/1/09

This page discusses the types of water that are available for use in aquariums and ponds and also for drinking for both animals and people. Also, check out these pages on my site relevant to water and water chemistry.

Water Chemistry
Water Comparison Table

On This Page:

Types of Water - tap, well, spring, bottled, distilled, mineral, RO, carbonated, and rain waters
Carbonation, Oxygenation, and Compressed Gases

I wrote this page off the top of my head (yep, no references, all original) on 11/6/06. I am an analytical chemist so most of what I wrote should be right. I hope you can understand it!

Types of Water

Check out my Water Comparison Table as well.

There are many kinds of water that are available for use in aquariums and ponds and for drinking. Many people have contacted me about their use of bottled waters for aquariums. In most cases, tap water will work for aquariums but there are exceptions. For any water source that you are using, it is a good idea to first test it for the following at least: pH, hardness, and alkalinity. See my water chemistry page for details on what those are. It is also a good idea to know from whence the water came and what has happened to it along the way.

Tap Water:

Tap water can refer to any water from a tap but I am going to refer to it as city water that has been treated. Such water usually comes from a reservoir but sometimes from wells. It goes to a water treatment plant where they continually test the water and add chemicals to it. They will add chlorine or chloramine to kill of bacteria that would otherwise cause their customers to possibly have vomiting or diarrhea. See more on chlorine below. They will add fluoride to help with teeth health. And, they will often add buffers and other chemicals to maintain the pH near neutral or slightly basic. Tap or city water has been made to suit the needs of people, not fish. Fish cannot tolerate chlorine so a dechlorinator is required when using tap water (see below under chlorination). Tap water may otherwise be okay to use. Some tap water may be hard but it varies on where you live. If the water tests out fine, it can be used for most everything. You should contact your local water treatment facility or Department of Public Works and request a water report if they do not already send you one. This covers dozens of tests that they do. They are required to send you a report if you ask.

Well Water:

Well water comes from the ground. It is usually of good quality (no bacteria). Because well water has no added chemicals, it can be great for aquariums and ponds. There are some areas of concern though. Some wells put out water that is slightly acidic or neutral and soft like the one I have but some wells may have high pH, hard water. In some cases, it is too hard for aquarium use and should be diluted or replaced with water sources that are lower in hardness. Another factor with water from a well that I have found is that it usually has some compressed gases in it and is low in oxygen. For that reason, I never change more than 50% of the water at any one time and aerate it well. The compressed gases (mostly carbon dioxide) come out of solution as tiny bubbles all over everything. Tap water can also have this problem. Some well water may be contaminated from runoff. For example, if one lives near a farm or source of contamination, high levels of pesticides, herbicides, fertilizer, nitrates, etc. may show up. If one lives near a landfill, then organics from various solvents and so forth may show up. Well water should be tested by the health department regularly (every year or two). If you live near a dump like we do, that is free. Our water is, so far, free of contaminants.

Spring Water:

Bottled spring water comes from a spring out of the ground. It is similar to well water but tends to be less hard (in those areas prone to high hardness). You can get actual spring water from a spring as well but without testing the water, you do not know if it is safe as far as chemicals or bacteria. Spring water is the best choice for a bottled water for soft water fish if your tap or well water is not appropriate.

Bottled Water:

If you buy bottled water that does not say it is spring water, distilled water, mineral water, etc., then guess what it is? It is most likely just tap water from some other location! It may therefore have been chlorinated and fluorinated. Some bottled water may have been filtered and be called filtered water. Filtered water should not have chlorine or other toxins and would be better.

Distilled Water:

Distilled water is water that is nothing but water. That means it has no ions, no minerals, and no hardness. Without anything to buffer it, the pH is not very stable (although technically neutral at a pH of 7). Distilled water is obtained by distilling (boiling) water and collecting the steam and re-condensing that. Distilled water is NOT safe by itself to be used with animals or for drinking. It will suck the ions out of any animal. Distilled water can be mixed with other waters and used. By mixing it with hard water for example, a water with lower hardness can be obtained. Distilled water is mostly sold for cleaning contact lenses because it is just water. There are a few cases where distilled water can be used as is for animals. Certain crustaceans such as triops and fairy shrimp can be hatched in distilled water.

Mineral Water:

With bottled mineral water, you cannot be sure what you are really getting. If it is pure enough and just water and minerals, it could possibly be used to raise the hardness of a water mix. Do not use mineral water if it is carbonated or has other ingredients.

Reverse Osmosis Water:

So, what do you do if your tap or well water is hard but you have soft water fish? If you don't have the soft water fish yet, reconsider. Otherwise, the best thing to do long term is to buy a reverse osmosis system. Reverse osmosis or RO uses a membrane to extract the ions from water to reduce the hardness. When the hardness goes down, the pH and alkalinity normally follow. RO water is similar in some ways to distilled water except that the filter has a limit to the membrane size so that some smaller things are still going to be in the RO water. RO water is commonly used with discus tanks for example or to be mixed with hard water to bring it down to a useable level. RO systems are expensive initially.

Carbonated Water:

Ok, why did I add that in here? I just did to try to make the list complete as to the types of water that one may encounter. Carbonated water is NOT suitable for use in aquariums or ponds or for drinking for animals other than humans. If carbonated water is pure carbonated water, it could technically be heavily aerated to drive off the carbonation and then be used in an emergency.

Rain Water:

Rain water is nature's water supply. It is close to being distilled water as far as its attributes. It can, however, pick up contaminants in the atmosphere such as sulfides or acids that lower the pH. As with distilled water, rain water is low in hardness and alkalinity and does not have a stable pH. Like distilled water and RO water, it can be mixed with other waters that are hard to reduce the hardness. Unlike distilled water and RO water, rain water is usually pretty safe to drink (at least for animals).


I am sure the history of chlorination is interesting but I will not go into that here. Suffice it to say that someone decided that adding chlorine to water was a good way to kill the bacteria found in water sources that can lead to such things as vomiting and diarrhea. Since we really do not want those things, chlorination really is something that we need. But, while chlorine is toxic to bacteria, it is also toxic to other living things including fish, shrimp, snails, our pets, and even us at high enough doses. If tap or city water is not treated, the chlorine will erode the gills of our piscine friends so that they cannot breathe. The fish will gasp and meet an untimely demise. Luckily, the chemical sodium thiosulfate, when added to tap water will render the chlorine non- toxic so that we can keep aquarium fish and ponds. I refer to additives with that chemical as dechlorinator but they may also be called tap water conditioner. Some experts say that if you are changing 10% or less of a pond, that you do not need to worry about dechlorinating. I think I would anyway just to be careful. Dechlorinator can be added to ponds or aquariums as the water is going in or maybe half before adding the water and half after to cover things. Biological filters should not be cleaned with chlorinated water as it will kill off some of the good bacteria.

I think it is also a good idea for those of you with tap/city water to filter your (and your pets) drinking water through carbon to remove the chlorine (unless you have gone to bottled water).

In the past, people were told to put out water for a week and then use that water for their next water change. This came about because with aeration and/or time, chlorine will leave the water as chlorine gas. For ponds, one idea was to squirt the water across the surface to not only release the chlorine as gas but to add more oxygen to the water. Since many water plants now use chloramine, these old ways will not work for those waters.

At some point in the magical history of chlorination, someone realized that chlorine is not all that stable in water, hence, that bleach smell from your tap. They decided that the chemical chloramine was a better bet. It lasts longer and kills those bad bacteria for a longer period of time. This was not good news for our fishy buddies. Chloramine will not go away by letting water sit or by aeration. When dechlorinator is added to chloramine, the chlorine is deactivated but ammonia is released. Ammonia is also highly toxic to fish. So, if your tap water has chloramine (call the water company in your area to find out), be sure to add a tap water conditioner that says it treats for chloramine which means that it not only breaks the chlorine- ammonia bond and deactivates the chlorine but also deactivates the ammonia. Both chlorine and ammonia burn the fish's gills and make it hard for him/her to breathe.

I am so glad that I have well water!


Fluoride is added to most tap water sources as well as chlorine. Fluoride fights cavities. Unfortunately, it is also toxic. It is not good for humans or pets to drink it. It is not good for fish, shrimp, snails, etc. to be exposed to it. At the same time, it is highly unstable. If you put out a glass of water, by the next day, most fluoride is gone. As a chemist, by the time I get a sample for fluoride, I cannot find any. If you just look at the water, the fluoride is gone. Nonetheless, it is still a good idea to filter tap water for drinking purposes for people and pets. Adding carbon to an aquarium filter should remove what little is in there. Heavy aeration will help drive it off. Ponds are normally large enough that fluoride is irrelevant.

Carbonation, Oxygenation, and Compressed Gases

We are all familiar with soda. Soda is a carbonated beverage. That means that carbon dioxide is bubbled under pressure through the liquid. When carbon dioxide is in water, it produces carbonic acid. Soda has a pH that is very low, sometimes as low as 1 or 2 but it also has phosphoric acid. Purely carbonated water will have an acidic pH as well, maybe 3 to 5. When released to room pressure, the carbon dioxide comes out of solution as bubbles.

When water is compressed under the ground or in pipes, something similar happens but much less drastically. Well water and sometimes tap water may have some compressed gases in it (combinations of carbon dioxide and nitrogen mostly). This is because there is more pressure underground or in pipes. When added to a glass or aquarium, little bubbles may be seen as these gases come out of solution. For us, it just tickles our fancy. For aquarium fish, it can be deadly. Just as divers who come up to the surface too quickly can get the bends where gases come out of solution in their blood, so too can fish have something similar happen to them. Compressed gases can come out of solution inside the fish. This is called gas bubble disease. For this reason, water changes should not be more than 50% at a time (more only in emergencies) and water should especially be heavily aerated following water changes. When setting up a new tank or pond with 100% new water, it is best to aerate for at least a day before adding animals. This is one reason why. Aeration is best accomplished with an air stone attached via tubing to an air pump.

Another reason why aeration is important is that when water has a lot of carbonation or compressed gases, oxygen is usually not one of them. Water straight from a well (and sometimes a tap source) does not have enough oxygen as is to support fish for long. Once aerated, it is fine. So, pull out those air pumps!

Yet another reason to aerate is because of that carbonic acid. When you measure the pH of straight well or tap water and then aerate it and re-measure the pH a few hours later, you will most likely see an increase in the pH as the carbonic acid leaves the water as carbon dioxide gas. Any pH adjustments that one might need to do to aquarium or pond water should thus be done only after new water has been thoroughly aerated to equilibrate the pH.

There is a saturation level for each gas based on temperature and pressure. That is the point at which you cannot jam more of the gas in question into the liquid no matter what you do. Increasing the pressure or lowering the temperature allows you to jam more gas in there. That is why cold water holds more oxygen (or other gases). That is why on a hot day, you had better watch that that soda bottle does not explode in your face after you open it! It is also why fish in ponds can suffocate on a very hot day even with some aeration.

They sell inexpensive test kits for oxygen levels. Here is one:

Oxygen and Aeration in Ponds:

Perhaps the most overlooked part of a fish pond is aeration. Most fountains and waterfalls provide aeration, and often, that is enough. But, if the pond is overstocked, small, shallow, or it is really hot outside, the oxygen levels may go low enough to cause problems. If the oxygen is just a little low, the fish will gasp under the waterfall or any other moving water. Once the oxygen levels get too low, the fish die. Sometimes only one fish will die. Sometimes, the entire pond dies off. For this reason, I suggest that most any fish pond over 200 gallons have a separate air stone with air pump in addition to the usual filter and water movement. In order of tolerance (most to least) of low oxygen are mosquito fish, rosy red minnows, goldfish, koi, and orfe. That means that orfe need more oxygen than say goldfish. Also, larger fish need more oxygen so they may be the first to die in low oxygen levels. You can buy an oxygen test kit to measure oxygen levels. At any given temperature, water can only hold so much oxygen. The warmer the water is, the less oxygen it can hold. So, if a pond has a lot of fish, and the water is 90 degrees F, even with aeration, the oxygen may drop low enough to harm fish. On hot days, added shade (plants or suspended tarps) and/or partial water changes (with added dechlorinator for those with city water) may help lower the temperature and increase oxygen levels. Many people have told me that they run their waterfall, fountain, etc. during the day and turn it off at night. They want to know why their fish died. Animals always use oxygen and output carbon dioxide. Plants take up carbon dioxide and release oxygen during the day but, at night, the process is reversed. That means, at night, plants use up oxygen and give up carbon dioxide. In a hot pond with a lot of animals and/or a lot of plants under the water, the oxygen can plummet to dangerous levels at night leading to fish gasping in the morning. So, run your pump all day and get an aerator if you do not have one!

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