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Tests are calibrates to read in certain values. Some tests are pretty much always done using only certain units. Some tests can be calibrated in any of several ways.

The most common tests done in aquarium keeping are:

Ammonia. Most tests do not distinguish between ammonia (NH3) and ammonium (NH4+)
Ammonia is toxic to fish, and when their body makes ammonia, they get rid of it though their gills. Also, decomposing poop, fallen food, fallen leaves can produce ammonia as they decompose.
There are beneficial bacteria that remove ammonia from the water and turn it into nitrite. In a new tank there are almost none of these bacteria, so you cannot add fish. The fish would poison themselves. Do a fishless cycle to grow the beneficial bacteria, and learn about using test kits without harming fish.
Fish are sensitive to ammonia at different levels. Fry are generally more sensitive than adults. Some species are more sensitive than others. The general recommendation is to keep the ammonia under .25 parts per million, and zero is best. Do water changes as needed to keep the ammonia level down if something has happened to the beneficial bacteria that remove it.
Plants can use ammonia or ammonium as a source of nitrogen, as a fertilizer.
Ammonia is measured with several different tests, but always is reported in ppm or mg/l.
Parts per million and milligrams per liter are the same thing.
There can be problems with some test kits and some dechlorinators. Make sure the test kit and the dechlorinator are compatible, otherwise you may get false readings from the test kit.

Nitrite, NO2 is produced by certain bacteria that remove ammonia. It is toxic to fish, causing a problem called Brown Blood Disease. NO2 gets into the blood and makes it not able to carry oxygen. Fish can get stressed, and can die.
There are beneficial bacteria that can remove NO2 from the water and they turn it into nitrate. Do a fishless cycle in the tank to grow these bacteria before you add fish. Plants can use NO2 as a source of nitrogen.
Different species of fish and different ages of fish are more sensitive to NO2. General recommendation is to keep the NO2 under 1 ppm if something happens and the beneficial bacteria cannot remove it.

Nitrate, NO3 is the end product of the nitrifying bacteria. NO3 is stressful to fish, but not deadly in low levels. Plants can use nitrate as a source of nitrogen. General recommendations vary. I have seen tests that suggest 40 ppm is at the top of the 'safe' level. In my tanks the fish are stressed at that level. I keep the NO3 between 5-10 ppm most of the time, and do water changes if it gets closer to 20 ppm. I ADD it to the tank if it gets under 5 ppm, which suggests the plants are starving for nitrogen.

GH is a measure of Calcium and Magnesium. This is the most important reading when you are calling the water Hard or Soft, and trying to figure out what kind of fish to keep. Fish from soft water rivers and lakes prefer a low level of minerals in the water. Plants are not usually very particular, but Ca and Mg are necessary nutrients for plants, and should not be allowed to get too low. Some plants will only thrive in water with very low calcium levels, but these are usually only available through aquarium plant clubs, so you can ask about them before you buy them.

KH is a measure of carbonates in the water. Carbonates act like a buffer that will stabilize pH. Low KH means the pH is more likely to be low, and can vary more easily, depending on what else is in the tank. High KH usually means the pH will also be high, and is harder to change the pH. Some plants (about half the species we grow in aquariums) can use KH as a source of carbon. They can only do this if the water is depleted of CO2.

GH and KH are measured most commonly in either of 2 ways. Some kits report ppm (same as mg/l) and some kits report the results in German Degrees of Hardness. 1 German Degree of Hardness = 17.9 ppm.

pH is a measurement that is usually given without units. It is a relative level of H+ and OH- in the water. When there is more H+ the water is acidic, and the pH will be under 7.0. When there is more OH- the water is alkaline, or basic. The pH will be over 7.0.
Before fishkeepers had tests to test mineral levels in the water, all they had was pH. It turns out that the mineral level is much more important than pH. Fish can handle some pretty wild changes in pH, as long as the mineral level does not change. For example, adding CO2 to the tank almost always drops the pH. The fish do not mind, because the mineral level (GH) has stayed the same.

TDS is a test of all the stuff in the water. It does not tell you what the stuff is. It will tell you that there are minerals and salts in the water and report the readings in any of several ways. Mine reports it as if all the stuff was salt (sodium chloride) and tells me parts per million. Some people say that is the wrong way to test and report the results, but I can use it when acclimating fish, so it works for what I use it for. Even more important than GH, if you test the TDS of the water the fish are in, and in the new water you can tell if the fish will acclimate to the new water quickly (very little difference) or if you need to take longer to move the fish from one place to another. (Greater difference in TDS.) When I buy new fish I put them in a quarantine tank with water that as closely as possible is equal to the GH, KH and TDS of the water they were in.
Fish that evolved in soft water, low mineral levels thrive in water with low TDS.
Fish that evolved in hard water, high mineral levels thrive in water with high TDS.

Other tests:
You can buy test kits for many of the fertilizers (Fe, P, K...) that plants need. The easiest test is to look at the plants. If they look deficient in something, then add that fertilizer or mineral and see if it helps.

11,721 Posts
Here is a quick way to get some idea of what you are dealing with:
Test the tap water before you set up the tank.
If the GH and KH are low, you can do anything you want, perhaps adding minerals to make it a hard water tank, or not adding minerals, using the tap water as it is, and running a soft water tank. Either option is easy to do.
If the GH and KH are pretty high, then you will find it easier to run a hard water tank. It is possible to remove the minerals and make it soft water, but not as easy.

Post all the test results here, and lets see what you have in the tap!
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