I saved Diana's post on this. Maybe it will help.
From this thread -
"Plants use many elements to grow.
Some they take in as simple molecules.
They use different elements in different amounts.
Aquariums supply these elements in different ways.
Hydrogen, oxygen, carbon make up almost all of a plant.
Nitrogen, phosphorus, potassium are the next-most common elements found in plants.
Calcium, magnesium and iron are next.
There are about a dozen more, but are needed in very small amounts.
Plants get hydrogen and oxygen with no help from us. (well, you gotta top off the tanks now and then)
Carbon is best taken up by the plants in the form of carbon dioxide. Some plants can get the carbon out of carbonates in the water. In the air (house and garden plants) CO2 is no problem. But only a certain amount of CO2 will dissolve in the water, so aquarium plants generally need a supplement of carbon.
These are the fertilizers often called Macros:
N (nitrogen) enters the tank as fish food. The protein has nitrogen. The fish eat the food, or bacteria eat it, and the protein shows up in the water as ammonia. Plants can use ammonia. Other bacteria can also use ammonia, turning it into nitrite and then nitrate. Plants can use these, too. If there are enough fish (meaning you are adding enough fish food to the tank) then you might not need to add nitrogen to the tank. Test the NO3. If the nitrate seems to stay about 5-10 ppm, there is probably enough N for the plants. If you have to keep doing water changes to lower the NO3 then there is certainly enough for the plants.
P (Phosphorus) enters the tank with fish food, too. Plants take up the phosphate form, and this is pretty common in the tank. Similar to nitrogen, if you have lots of fish there is likely enough P entering the tank. There are separate tests for P, but as a start, if the NO3 is OK from fish food, then the P is probably OK, too.
K (potassium) is one of the elements that plants need quite a bit of, but there is not much in fish food. Even in a low tech set up it is often needed to dose K. The last I heard tests available to most aquarists for K are not very reliable. Excess K in the tank is not a problem.
These are the fertilizers often called Micros:
Ca (Calcium) is used in amounts that are less than the macros, but pretty high for a micro. Generally present in water with a General Hardness over 3 German Degrees of Hardness. Not always, though. Usually present in tap water.
Mg (Magnesium) is also used in amounts that are less than macros, but pretty high to be called a micro. This one also is usually present in water with a GH over 3 German Degrees of Hardness. Usually present in tap water.
If the GH is under 3 degrees then there is some question if there is enough Ca and Mg present, and if they are in the right ratio for plants. Supplementing with a GH booster is a good idea. If the GH maintains a fairly stable value around 4-5 German degrees of hardness, then you are probably adding enough Ca and Mg with each water change to keep the plants, bacteria and fish happy. (Fish utilize some minerals from the water, too)
Note: Some tap water is weird, and supplies one or the other of these minerals, or supplies them in an odd ratio that is not good for the plants. Don't worry about this when you are getting going (as long as the GH is over 3 degrees), but if some symptoms of deficiency show up after you have the other things squared away it is probably worth testing the water for Ca and comparing it to the GH test to see what the story is.
Fe (iron) is the other micro that plants seem to use a lot of. This one is a bit tricky, because some molecules that contain iron are easy for plants to take up and use, but other forms cannot be used by plants. Also, Fe is not usually present in enough quantity in fish food. Most people who need to dose iron use a form called Chelated Iron where the iron is bound in a molecule that keeps it available for the plants to use. Often this and potassium are the elements that are needed for even the low tech tanks.
All other micros: Most fish foods contain a little bit of all these, and they are needed in such small amounts that a low tech tank probably does not need them to be added as fertilizer. When they are needed, many aquarists will dose them in a blend that includes iron, then test just for iron. By keeping the iron at the right level it is assumed that the other micros are good, too.
Substrates can latch on to some of these elements and molecules and hold them in a way that plants can get them when they need them. Substrate with high Cationic Exchange Capacity (CEC) is very good.
Low tech is a tank with low light, no CO2, or maybe DIY yeast, substrate that may hold the fertilizers (high CEC) but is not really a rich source of nutrients. You might use a slow release fertilizer tablet buried deep in the substrate.
This style of tank usually needs K and Fe, and adding carbon in the form of Excel or DIY/Yeast is usually enough to boost the plants into looking pretty good.
High tech is a tank with high light, the plants are growing a lot faster, so demand more fertilizers (macros and micros) to support that growth. Often the CO2 is supplied via a pressurized system. The substrate is definitely high CEC. People who keep this sort of tank are right on top of the fertilizer needs of the plants, and may use a system such as Estimative Index to make sure the plants do not run out of anything they need.
Fertilizers are generally available in 2 forms.
Liquid: You are paying for shipping water. The amount of actual fertilizer is quite low. It is a good way to get started fertilizing the tank, but then save the bottle and refill with dry fertilizers that you mix with water.
Dry, Tablets, pre-mixes: Well, someone else has done the work, but these can be better priced than shipping water. Tablets or other slow release fertilizers are a good way to dose the tank. Every month or two push some pellets deep in the substrate. If the fertilizers in a blend are the right ratio for your tank, that is OK, but often the ratios are not right, so you end up over dosing something to get more of what is needed, or else buying another product to make up for deficiencies in the 'one size fits all' blend.
Dry, agricultural grade powder: The best buy in fertilizer. You are paying for exactly the elements or molecules that your plants need. Each nutrient is separate (mostly) so you dose just what the plants need. Some of these can be added to the tank directly, dry. Some are better if you dissolve them in water first. You can re-create the mix that you bought as liquid, and dose it the same way. High CEC substrates will remove some of these from the water to make a long-term supply for the plants, evening out any irregularities in dosing."
Easily found bottles at the big box pet stores are potassium [K] and a little of the micros, sometimes just iron. If your tank is lower light with no carbon dioxide then likely one of those bottles will suit you just fine. Better LFS will carry macros in separate bottles and will be labeled as such. Any fertilizer label will likely suggest lower dosing than our higher lighted tanks need and it is just fine to increase the dosing over what the label reads of a fertilizer. The exception is Seachem Excel. It is NOT an NPK fertilizer but a carbon source which is toxic if over dosed. Any product that is a carbon source needs to be used according to the label.