Reverse osmosis is a process that will ideally produce pure water. In simple terms, water is forced through a membrane at high pressure to withhold any impurities and allowing only pure water to pass through. Models vary, but units commonly incorporate two membranes, one allowing small impurities and water to pass through and the other removing nearly all impurities to produce a "pure" water. I put quotations around the word pure, as most units available do not actually produce a 100% pure water, cheap units may only slightly improve water quality depending on a range of factors. Deionization is a process of removing mineral ions, such as sodium, calcium, and chloride to name a few. Water filtered by Reverse Osmosis is further purified when filtered through a deionization unit, which are included together on some models.

Reverse Osmosis and Deionization Units are usually for the advanced aquarist. Hobbyists keeping sensitive fish such as Discus, or other soft water fish utilize these units to produce their own RO/DI water rather than buying it to save money. The average hobbyist does not necessarily need an RO/DI unit unless your source water is especially poor.

Using an RO/DI unit generally lowers the pH of the water produced. This makes it simple for hobbyists to create ideal water parameters by using chemical products widely available on the market today.

Water conditioner is nothing new, it has been on the market for years, and like any product available for such a long period of time, many manufacturers have their own line of water conditioners. Water conditioners are most often a liquid chemical added to the aquarium water during water changes or when adding new fish to prevent stress.

Many people say all the tap water conditioners are the same, this is not true. Municipal water supplies commonly use one of two chemicals to sterilize the water supply: chlorine, and chloramines. Most water conditioners will remove these chemicals, but not all of them. It is a wise idea to purchase a water conditioner that will remove both chlorine and chloramines, since it is not uncommon for municipal water supplies to switch between the two.

Most of the water conditioners will claim to have added benefits, but these are usually minimal, so the key here is to remember why you purchased the product in the first place - to remove chlorine and chloramines from the water.

Water conditioner is necessary if you will be using water from a municipal water supply to maintain your aquariums even if you use an RO/DI unit. If the chemicals used in the water supply are not removed, they will severely stress your fish, and could lead to death. Another point of adding conditioner to your water is to prevent stressing your fish when performing water changes or adding new fish. This point is hotly debated in the community, some will say this fact is true others will say this is simply a sales pitch. The decision is ultimately yours depending where you stand in the debate.

Distilling water is a complex, and slow process. Distiller units are expensive, and are not economical for even average aquarium setups. Many people also think distilled water is completely pure, but this is not the case. Ammonia (NH4), a toxic chemical for fish is present as a gas dissolved in water. Therefore, the distillation process does not completely remove ammonia, so a chemical is needed to remove the ammonia for water to make it safe for fish, which is even more money not well spent.

Bottled water is an economical decision for those with one or two small aquariums or fish bowls. Distilled water is not acceptable for reasons mentioned above, so purchasing regular bottled drinking water is what we will refer to here. For small aquariums (1 or 2 gallons), a weekly water change of a half gallon is not entirely expensive, and very convenient for fish-bowl owners. The water has been filtered before bottling, so the addition of water conditioner is not necessary.

This water is recommended for hobbyists whose source water comes directly from a private well that produces water of good quality. Beware however, that in most areas of the United States, well water has a very high pH, unsuitable for sensitive fish such as a Discus. For hardy fish, such as mollies and guppies, water from the tap is perfectly suitable as long as the source water is from your own private well producing water of good quality.

If you would like to have the quality of your water tested, a good place to enquire about water testing would be a nearby university. Most universities have science programs that will test your water and provide you with great detail about the composition of your water for a reasonable rate.

A good place to look for help is a friend in your neighborhood who keeps the same type of fish you would like to keep. Ask them about their water conditioning methods, they will be likely to share with you excellent tips and tricks about conditioning the local water supply. You can also ask an honest employee at a reputable aquarium shop, but beware of those stores that are looking to make a quick buck and paint the scarier picture.

As always, research the fish you would like to keep ahead of time, this saves you money, and prevents future headaches. You owe it to yourself and to your fish.