Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Tap water varies.
Test your tap water with all the tests you have.
Ammonia may show up if your water company adds chloramine. Mine shows 1 ppm. (Contra Costa Water District) Dechlorinator will (should) lock up this ammonia. It is still available to nitrifying bacteria, and they will turn it into a small amount of nitrate, which the plants can also use.
Nitrite might be there. I hope not, it is a toxin to mammals as well as fish.
Nitrate may be in tap water. A friend's well in an agricultural area (Brentwood) tests 10 ppm NO3. This is enough to keep the aquatic plants fed for a day or two! My friends do not drink this water!
GH is a measurement that includes calcium and magnesium. Most average tap water, coming from the ground somewhere along its path will have some of each. Your tap water may be weird, or if you are using reverse osmosis or distilled or rain water the GH will be too low. GH of about 3 degrees is a minimum for most average aquarium plants, and some prefer harder water. Most soft water fish thrive in water with a GH between 3-9 degrees, some prefer it not higher than 5 degrees. If your tap water is much harder, GH from about 9 degrees on up certain fish prefer it, and would be a better choice, but most plants do not care. It is OK for them.
KH is carbonates. This is more interesting as a buffer that stabilizes pH, but some plants can also use it as a source of carbon. If you are adding carbon from pressurized CO2 then it is better to have a little more KH in the tank to stablize the pH. 3-5 degrees is good. For hard water fish and plants the level can be much higher, and this usually raises the pH higher.
These two measures of hardness vary with the origin of the water. Hetch Hetchy system starts in rock that is a lot of granite, which does not dissolve in the water. (San Francisco water is very soft)
The water that flows down from melting snow in the Sierras picks up some of the minerals from the mountains, and may pick up some agricultural chemicals from the valley. East Bay MUD seems to have somewhat softer water than Contra Costa Water District, but the water in some parts of San Jose is much harder.
pH does not mean much as far as fertilizers go. At extremes of pH some fertilizers may be bound up with other elements and not available to the plants. (But we do not usually keep aquariums at these extreme pH levels) Fish do not usually have a problem with changes in pH such as from adding CO2, as long as the full range of that change is within their tolerance for pH. Many water companies will add something alkaline to the water to change the pH. Acidic water can be very hard on the pipes.
Contra Costa water, for example has a pH varying from the upper 7s to the upper 8s. (Sodium hydroxide- I asked them) A friend measured the pH at 9 last summer, but the KH was only 4 or 5 degrees. (One of the water companies that starts with Contra Costa Water.)
Iron, Phosphate and other tests can also show levels of other plant nutrients that may be found in tap water. A pretty easy test for iron is to look at the bathroom fixtures. If they have red or rusty colors when they should be white there is iron in the water. Some relatives in Gold Run could not use their well water even for washing because of the high iron. All the white towels turned pink!
Overall, tap water may have some nutrients, but not much. Sometimes it is high in one thing or another, even to having toxic levels of something. (A client in Clayton had toxic levels of Boron in their well water. Landscape plants were dying)
One water change a week, with 'not much' means that tap water is not usually a really good source of most fertilizers. If you are running a low tech system with a month or more between water changes, forget it. Water is not a good source of fertilizers. If you are running a flow through system, or more frequent water changes, then 'not much' may really mean there is a nice level of something to help the plants, and it is being replaced fast enough that the plants are not deficient.
However, tap water is different, well water varies... the only way to know what your water is like is to test it.
Water companies print annual reports and these are available on line. Look up the report for the water company that supplies your water. If you are on a well it can be tested professionally, but simply using all the aquarium tests is a good place to start.