Different Sources of Water for Your Fish Tank
(The First Tank Guide)
What kind of water should I use in my aquarium?
You have several choices: tap water, water from another tank, Reverse Osmosis or De-Ionized purified water, softened water, bottled water, rain water, or collected water.
The easiest and most obvious - and usually the cheapest - is to use tap water. Tap water is available to almost anyone keeping an aquarium, and is almost always going to be fine for your fish. However, you do need to be aware that if your tap water is from a municipal water supply, or if you have a holding tank for your water supply, you probably have chlorinated water, and you will need to use a dechlorinator or a chloramine remover to make the water safe for your aquarium, depending on which compound is used in your water system.
Water from Another Aquarium
Water from another aquarium is another possibility. This can sometimes be helpful when setting up a new tank, but it has many drawbacks. If you do not know the history of the tank and what the water conditions really are in this tank, this can pose a risk of disease or chemical toxicity to your fish. This is especially true of water from tanks in pet stores where rapid turn-over of fish can introduce a disease into the water with no opportunity for the store employees to detect the disease. Additionally, the benefits that presents to the cycling process are minimal. Between the risks of this behavior and the minimal benefits, this is not recommended. The exception to this is when moving a used tank, where taking as much of the water from that tank as possible dramatically helps the biological filter get restarted, if the filter was live and operational before moving the used tank.
Reverse Osmosis or De-Ionized Water
RO or DI water purchased from a pet store is another option. RO and DI are two methods of purifying water. This removes the impurities and any particulate that could be in the water, or neutralizes the pH by eliminating the various ions that could be floating in the water. While this gives you very pure water, this water is potentially dangerous for your aquarium. With all the impurities removed from the water, the water's ability to maintain its chemical properties are compromised. This presents a risk that the water could make sudden and dramatic changes in pH, alkalinity, and hardness when exposed to any kind of chemical - even the aquarium equipment. It is important to know how to balance this water appropriately if you get it to make sure it will stay stable. In addition, RO or DI water will have most, if not all, of the trace elements necessary for your fish and plants to remain healthy removed, further prosing a risk to your fish. It is important to get the correct supplements for your fish or plants, and know how to add them to the water and how to test that you have the correct concentrations. Though RO or DI water can be very useful to advanced aquariists with the experience and knowledge of the water conditions they are trying to duplicate and how to duplicate them and test that the water is still safe, I would not recommend RO or DI water to a beginning fish keeper.
Many people want to use softened water from a home water softener. Though this should be safe, you should check with the manufacturer of your softener or the softening media you use to confirm that the water is safe for aquarium use before using it.
Bottled water is another choice people consider. I would recommend against bottled water unless you KNOW your tap water is unsafe. Bottled water is often not regulated or tested, so the water parameters are unknown. Also, bottled water often has vitamins, flavors, preservatives, colors, or other additives added to make the water more appealing. These can be devastating to your fish tank.
Some people are interested in using rain water for their aquaria. In this case, you need to consider the condition of the air and the condition that leaves the rain water in. Tap water has been filtered through soil and then processed to insure that it is potable. Rainwater can carry pollutants which it has washed out of the air, as well as possible contaminants from tubing, gutters, piping, roofs, and other surfaces it has flowed through or over before getting to your collection point - as well as risks of contamination in the collection apparatus. Rainwater is also almost always lacking in necessary trace elements to keep the fish healthy, as well as those necessary to help the water maintain chemical balance.
Some people want to just collect water from a local stream or lake. Though tempting, this poses several risks. First, this water could contain pollutants which may not cause significant problems while they are washed downstream, may cause problems for your fish which would be likely to have prolonged exposure. In addition, this water is likely carrying pathogens for diseases which your fish are ill prepared, which could result in a disease epidemic in your fish tank. It is also possible to pick up parasites which could wipe out your fish. In addition to parasites, you may pick up some native plants or animals which, in turn, may be illegal to keep in your area.
My Recommendation: Tap Water
My recommendation for any beginning aquariist would be to use tap water unless you know there is a reason to not use your tap water, such as your local municipal water supply is know to be contaminated with some toxin. In general, for a beginner, tap water will be the best choice. It should be excellent for a wide variety of fish, is readily available, is cheap, and is easy to prepare for use in your fish tank (you just need to remember to dechlorinate). Tap water usually has known and relatively stable parameters, and tends to be reasonably chemically stable, so sudden fluctuations in pH or other parameters are unlikely unless you start messing with them.