Fertilizing to Suit
We should start by choosing the overall plant growth rate of your particular tank. Stem plants are usually fast growers, while rooted plants tend to be slower growers. You will need to find out how fast your plants can grow! However, total plant mass and health have a significant impact on nutrient demands. I personally believe that it's too difficult to differentiate between growth rates of plants, but when plants are healthy, their uptake rates can really suprise you! When factoring in lighting, which acts as a catalyst for plant growth, it makes every tank different--while a certain plant may grow quickly for a few people, that very plant may grow much more slowly for others.
Let's view fertilizers like this: macro and micro. Micro is usually used minimally, while macro is used in much larger proportions. It also makes sense that when dealing with larger quantities/proportions, that larger problems may occur when dealt with improperly. So that's why we begin our fertilization methods with microelements, or "trace" fertilizers. We will also start out with the lowest dosages possible for the particular tank we are using. Everyday studying (i.e., relaxing) should be done in order differentiate between a plant doing well and one that has deficiencies. When sustaining plants only on water changes, CO2, light, and trace nutrients, it allows the aquarist to determine the contributions of the fish, their food, and their wastes. We can always attribute a certain amount of Nitrogen and Phosphorous due to fish food and their waste. When dumping in nutrients like they're going out of style, we aren't able to determine the effect of the fish, their food, and their waste. In a slower growing tank, this may play a critical role, but in a fast growing one, food and waste may be insignificant.
In essence, we want to test the waters...we want to give our plants and inch and let them try to take a mile. We will rely on a clean tank/water and the absolute necessities such as fish food (and their byproducts) to help the plants. We will progressively give the plants more trace elements. Eventually, we will begin to observe what our tank is not providing enough of (macro nutrients). After progressively increasing trace nutrients with no more positive results, it's time to turn to macro nutrients. Usually, algae will show up at this time, but in very lean conditions, we will have very mild algae issues. When a tank is saturated in one or more nutrients, it will be saturated in algae as well.
Trace Fertilizers
There are many to choose from, but for simplicity's sake, I prefer only two: Seachem's Flourish and Tropica's Plant Nutrition Liquid (PNL) because they are the most comprehensive on the market and both are priced well. Furthermore, these two fertilizers are separated by notsomuch as their chemical make-up, but rather their chelators (the agent that binds their elements together). Flourish uses more of an "organic" approach, and uses a relatively weak chelator. This means that plants can uptake the fertilizer faster, but the nutrients are not made available to the plants for quite as long. PNL uses a stronger chelator, meaning that while the plants cannot uptake the nutrients as quickly, the nutrients are made available to them for a longer period of time. Technically speaking, I would recommend Flourish for tanks dosed daily and soft water, while I would recommend PNL for slower growing tanks and/or hard water. Overall, I prefer PNL.
Iron
Iron (Fe) plays a large role in aquatic plant health and coloration. Usually in a high-light, CO2 injected tank where trace elements are added daily or semi-daily, you may find that there is no additional need for Iron. A lack of iron will be noticed in the form of pale plants. For example, hairgrass should be a medium-to-slightly dark green coloration when provided with enough iron. Hairgrass that is very light green in color, maybe with a yellow sheen, would indicate a lack of iron. The common sword plant (E. blehri) that lacks deep green leaf coloration indicates a lack of iron.
Again, there are two major forms of iron for the aquarist to choose from: Ferrous (Fe+2) and ferric (Fe+3). Iron in the ferrous state is much more easily taken up by the plants, and is the preferred form. Both Flourish and PNL use iron in the ferrous form, but as previously mentioned, they are separated by their chelators. Flourish uses a gluconate based chelator, while PNL uses a DPTA chelator. A weak chelator is ideal for the plants, as the iron would very readily available to the plant, but does not remain available to the plants for long--probably no longer than 8 hours. This type of iron would usually need to be dosed daily in order to have the greatest benefit. When dosing iron with a stronger chelator such as iron DPTA in PNL, the iron will be harder for the plant to use, but it will remain available for a longer time. This should be taken into consideration when accounting for your dosing habits. Iron in the ferric state is a little more rare, but will usually be found in dry iron supplements. I will not cover this type of iron for now, as for the average plant keeper, it's not as practical. Regardless, Iron is the biggest "little" element of all traces, and plays a vital role in the overall health of your plants and their vibrancy.
Macro Nutrients
Macro nutrients are referred to as such because they are used in the largest proportions in plants. They are Nitrogen (N), Phosphorous (P), and Potassium (K), or NPK. Nitrogen is by far the most used, with Potassium coming in second, and Phosphorous coming in last, but definitely not least. When plants are depleted of any of these nutrients, their growth will slow to a crawl, and the tank will result in algae growth. They can be purchased in liquid forms, but considering the dosing frequency in high-light CO2 injected tanks, and larger tanks in general, they are not the most cost-efficient form. Dry fertilizers are the widely becoming the most common form because they are the cheapest and many are combinations of the macro elements such as KNO3 (it provides K & N). Many chemical compositions are available.
Before getting into details of NPK, it's important to know what you are working with. By this I mean that when using tap water, you should know what's in it. Varying levels of each nutrient will exist in each individual's water. Once again, we will refer to our water report. Minimal levels will not be a much of a factor, but excessive levels may play a significant role in tailoring your fertilization needs. The most common problem would be excess levels of Phosphorous, but some may find Nitrogen an issue. Phosphorous is commonly added in urban areas to prevent pipe corrosion, but in rural areas, it may "naturally" occur from agricultural run-off. Be aware of what's going on in your area!
Nitrogen
Nitrogen, the staple of all plants--it helps the plant produce enzymes, proteins, and amino acids. It alone will have a significant impact upon plant growth. While nutrients such as P or K can hinder growth when limiting, a lack of Nitrogen will effectively halt growth and encourage plant decay. Fortunately, N is easily added. There are two types: Dissolved Organic Nitrogen (DON), and Dissolved Inorganic Nitrogen (DIN). DON is produced as you might guess, by bacterial oxidation from ammonia, resulting in NO3- from NH4+ (ammonia). DIN is your typical fertilizer supplement, such as from KNO3.
There is a significant difference in how the two types must be interpreted. It is fairly common knowledge that plants will use just about any form of N, if necessary, such as ammonia (NH4), nitrite (NO2), & nitrate (NO3). We think of this as the nitrogen cycle, and consider fish food/waste as plant food. However, in a high light, CO2 injected tank, this form (DON) of nitrogen is rather insignificant, and should not be relied upon as a significant source of N. It may play a more significant role in a low light planted tank with no CO2 injection. While plants may absorb DON, this is not a free pass to heavily stock fish, as you shall see.
Some sources may say that plants prefer ammonia, but this is simply not true per say. However, algae love it. It should also be known that algae can react far faster than plants to nutrient conditions. If you happen to have a condition where the production of NH4 is higher than the rate of oxidation by the biological filter and plant uptake, the result will be a constant excess of NH4. This excess NH4 will trigger algae growth in a matter of hours, and usually result in a major outbreak--most noticeably green water. The excess of NH4 does not need to be measurable, and in most cases, it isn't. Algae do not need much at all! This is one reason that using plant sticks such as Jobes or Miracle Grow are not recommended, as they contain high levels of NH4. This is also the reason that canister filters are recommended for the planted tank. Keeping a substantial harbor for bacteria will keep NH4 at bay. While they may not provide immediate bio-filtration "protection," they can easily compensate for fish growth and increased wastes over time. Power filters are notorious for their lack of biological filtration capacity, and are discouraged as the sole filtration method.
In a high light, CO2 injected tank, it is a given that DIN sources will be used. The most common is KNO3, because not only does it provide N, it also provides K! It can be bought as such, or is most commonly sold as "Tree Stump Remover," by Green Light. Plants will readily use this source of DIN without reserve. Another benefit to DIN is that it does not promote algae growth, unlike NH4. However, excessive levels will harm fish/shrimp, but these harmful levels far exceed this dosing regimen's requirements.