For those who have several larger aquariums, it can become expensive
to do water changes using the treatments that are on the market.
Often, this leads to putting off water changes, which is not good for
the health of the aquarium or its occupants. Often, the best solution is to try and find fish that prefer the water that your tap produces, but sometimes that isn't practical. Changing the pH can be done, but it is hard to maintain it very long as pH tends to drift as buffers are used up, fish produce carbon dioxide, and minerals leach out of gravel, rocks, etc. All these can affect the pH over time so that it must be readjusted. The best way to adjust the pH is with a water change. If done regularly, then the pH will not bounce up and down, which is bad for the fish. Below are a few ways to condition, treat, or adjust your tap water.
Add hardness, buffer pH:
Lower pH, soften water:
Remove chlorine:
Remove iron: Some people complain that there is too much iron from rusted pipes in their water, to the point that it settles out as a rusty sediment in their aquarium. The best way to treat for this is to fill a bucket with water and aerate it vigorously for a few hours with an airstone. Let it settle for several hours, and then gently pour the water from the top of the bucket. The aerating will cause the dissolved iron to oxygenate and settle to the bottom of the bucket.
Baking soda can be dissolved into the water to buffer the pH and add hardness to the water. You should try different amounts to obtain the best results for your water type and fish. This is only a temporary buffer, as carbon dioxide will use it up over time.
Adding coral, dolomite, limestone and other calcereous rocks to the aquarium will add buffers to the water that will raise the pH, and add hardness. This is good for marine, brackish, and African rift lake cichlid tanks.
Although phosphoric acid can be bought inexpensively at most aquarium shops, I highly suggest not using it. It provides a harsh pH adjustment that does not buffer to keep the water stable, and it will cause the water to "yo-yo" up and down, as well as promoting unwanted hair algae from the phosphates.
A more gentle way to lower pH, and soften the water at the same time, is to use peat extract. This can be done by simply placing peat, as can be found at any garden supply store, in a nylon stocking, tying it off, and adding it to your aquarium's filter box or placing it directly in the tank. Be sure that you use pure peat, not a fertilized combination such as "Sheep & Peat".
You could also try making "peat tea" by placing the nylon containing peat in a clean, well rinsed pot of water and boiling gently for about 15 minutes. Let this water cool, and add slowly to the water you plan to use. Measure the pH as you add it until you have the desired pH. Note that this will slightly darken the water. Some people dislike this, but most actually find it pleasant. It has a nice natural affect, and many fish come from waters that look like this. Discus, and many tetras and dwarf cichlids are among fish that appreciate this dark, rich water.
Most water conditioners contain the same basic compounds to treat the water. The one that all have in common is sodium thiosulphate. This chemical neutralizes chlorine by bonding the sodium with the chlorine to form small amounts of sodium chloride; common table salt. Sodium thiosulphate can be purchsed much more cheaply at drug stores or photography shops, where it is sold as a film developing agent (actually it is a developing stop agent). It can be used by adding just one or two crystals to a gallon of water. Stir well and the chlorine will be instantly neutralized. Note that this will not work on chloramines, which some areas use instead of chlorine.