Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Here is how each of the test kits relate to plant growth.
Ammonia- some plants cannot tolerate high ammonia levels such as may be produced by some substrates when they are first submerged. The ammonia is good when you are growing nitrifying bacteria, but needs to be kept at a level that will not damage plants. Some are OK only to 1ppm, many will handle 3-5ppm, a few are OK with higher levels, but the nitrifying bacteria do not like higher levels. An ammonia test will help you keep it stable, and under 5ppm.
Nitrite- Plants can use nitrite, but this may be of more concern when you are cycling the tank. Nitrifying bacteria do not like NO2 higher than about 5ppm.
Nitrate- Plants can use nitrate. Nitrate is produced by the nitrifying bacteria.
Phosphate- A plant nutrient. Generally plants use less phosphate than they do nitrogen, though I have seen suggested ratios all over the board. You might aim for 1 part phosphorus : 10 parts nitrogen. Most people do not buy a phosphate test unless there is some problem.
Potassium tests are either: unreliable (hobby level), expensive (lab quality) or both. Sure would be useful, though!
GH is a test of calcium and magnesium, combined. Both of these are minerals used by fish and plants. Plants use Ca and Mg in a ratio of about 4 parts Ca: 1 part Mg. You might be able to find this information from your water company. Fish thrive when the water hardness is in the right range, and the GH test is very useful for this, too.
Calcium test (without the magnesium) will allow you to calculate the level of magnesium. Just in case you cannot get the ratios from the water company, or are on a well (no water company). Most people do not bother with a calcium test unless there is a problem.
Iron is the only trace mineral that is usually tested for because of plants. You can get the other traces tested at a lab ($$$) or you may find other test kits (Copper, I am almost sure, perhaps others)
Chlorine may be useful if your water company alters the amount of chlorine or chloramine in the water. All water companies may do it. Some do this more than others. Nice to get a base line anyway. The dose of dechlor can be altered to handle the levels of chlorine or chloramine in the water.
pH is useful for several reasons.
If it is too high or low some nutrients are less available to the plants. Somewhere between 6.5 and 7.5 seems the best range for fertilizer availability.
If there is ammonia present, and the pH is high, then the ammonia is in the form of ammonia (NH3) which is quite toxic to the fish.
If there is ammonia present, and the pH is low, then the ammonia is in the form of ammonium (NH4+) which is less toxic to the fish.
By checking the pH through the day you can come to some conclusions about the CO2 levels in the water.
Some fish and other livestock prefer a certain range of pH.
Nitrifying bacteria grow best with the pH is well into the 7s to 8s. When you are cycling the tank you want the pH pretty high.
KH, carbonate hardness, is a test of how well the water can resist changes in pH. The most common buffer in the aquarium is bicarbonate and carbonate. High KH tends to keep the pH high, and resists changes. Low KH allows something else to determine the pH, and the pH is generally easier to change when the KH is low. Some plants can use the carbon from carbonates, and nitrifying bacteria get their carbon from carbonate.
TDS is Total Dissolved Solids. This is usually of more interest to fish and shrimp keepers, not so useful for plants. You can do enough water changes (frequency and volume) to keep the TDS in the preferred range for the livestock by testing the TDS.
There are several forms of tests. Not all tests are available in all these forms.
Tabs on a dry stick. Hold the stick in the tank water, remove it and compare the colored tabs to a chart. There is some question about the accuracy, but it is quick and easy to do. If you chart the results this is great, then you see a response that is different from the prior tests you know something is happening. There are 5-way, 6-way and single item tests. Ammonia is usually on its own stick. NO3, NO2, GH, KH, pH and chlorine are on the 5 or 6 way sticks.
Liquid reagent and test tube kits. May be more accurate than sticks, or might not be. You can get hobby level kits or lab quality ($$$)
Electronic pens that read certain things by dipping them in the water. pH is quite common, TDS is done this way, and several others can be found. There is a certain investment in the initial purchase, then you will need some standardized reagent that you will periodically check the unit with to be sure it is still accurate.