These nutrients are used in the greatest quantity by aquarium plants and are vital to many plant functions. Without them, plants would be unable to grow, repair, or maintain healthy tissue.

Calcium

Calcium is a vital element used by plants in the formation of cell wall structure and to maintain cell permeability. It may also activate some enzymes. Although calcium is present in sufficient quantities in most water supplies, it may be deficient if only rainwater or reverse osmosis water is used in the aquarium. Many gravel-based substrates (other than quartz substrates) contain some calcium and this, combined with at least a partial use of tap water, should provide sufficient quantities of calcium for the majority of plants. In most cases, calcium should not be added artificially to the aquarium, as an excess will limit the availability of other nutrients and raise water hardness. However, many plants from naturally hardwater areas will require higher levels of calcium. Due to the fact that it is readily available in their natural environment, these "hardwater" plants are not evolved to collect calcium efficiently in low-level conditions.

Carbon

Carbon is used by all living organisms as a basic structural "building block" and makes up 40-50% of a plant's dry biological mass. In terms of quantity, carbon is by far the most important nutrient. Plants obtain carbon from carbon dioxide (CO₂), which is broken down into oxygen (O₂) and carbon through the process of photosynthesis. Although plants need oxygen as well as carbon, the amount of oxygen required compared to carbon is minute, so the majority of oxygen is expelled as gas bubbles from the leaves. CO₂ is a gas, so the amount present in a body of water is affected by the air/water exchange. If a body of water is highly agitated at the surface, then the air/water gas exchange is increased and the level of CO₂ in the water will rise or drop, depending on the level of CO₂ in the immediate atmosphere. For plants to obtain enough CO₂ from the water, the level of CO₂ needs to be much higher than normal atmospheric levels allow. This means that it must be introduced into the water from an internal source (i.e. not from the surrounding air). In nature and in the aquarium, CO₂ is introduced into the water as a result of the breakdown of organic waste by bacteria and by plant and animal respiration. Plants can acquire the CO₂ they need by a number of methods, including direct uptake from the substrate through the roots, direct uptake from the water through leaves, "recycling" of respired CO₂, and through the breakdown of bicarbonates in the water. Although the concentration of CO₂ is highest in the substrate (due to the large amount of organic matter) it does not diffuse readily and therefore is not always available in large quantities in the immediate vicinity of the plant roots. The easiest way for plants to obtain CO₂ is directly from the surrounding water and through the leaves. In some aquariums, CO₂ levels are sufficient for good plant growth, although in most cases growth is limited by the amount of CO₂ present. Usually, it is necessary to introduce additional CO₂ to maximize photosynthesis and hence the amount of carbon available to the plants.

Hydrogen

Hydrogen is used as water (H₂O), mainly as a structural component to fill cells, provide support, and as a means of transporting properties throughout the plant. Clearly, hydrogen as H₂O is easily available in the aquarium and there is no need to add more.

Magnesium

Magnesium is a vital macronutrient for all plants with a part to play in numerous important functions, and an important ingredient in chlorophyll. Magnesium is also used to activate enzymes that form vital fats, oils, and starch. Magnesium is a "hardwater" nutrient and often found in levels proportionate to calcium levels. However, levels of magnesium in tap water vary a great deal depending on local conditions, so it is difficult to know whether additional fertilization is needed. Water authorities can often provide readings of quantities in the local tap water and test kits are available to measure levels of magnesium. The ideal level of magnesium in a planted aquarium should be about 5-25 mg/liter, although many plants live outside this range in nature. In general, there is usually sufficient magnesium in tap water in hardwater areas. Using a nutrient-rich substrate additive or soil-like substrate should provide a constant release of magnesium into the water. Alternatively, you can use liquid fertilizers, which are especially recommended for softwater aquariums. Many liquid fertilizers contain magnesium sulphate (better known as Epsom salts), which is ideal, as it provides both magnesium and sulphur. Bear in mind that an excess of magnesium in the water will inhibit the uptake of other nutrients, particularly potassium. In fact, potassium deficiency is often due to an excess of magnesium.

Nitrogen

Nitrogen is one of the major nutrients required by all plants, both aquatic and terrestrial, for strong growth and good health. It is used mainly in the production of proteins and nucleic acids and makes up about 1-2% of a plant's dry weight. Plants do not take up nitrogen in its "raw" gas state (N₂) but can obtain it in a number of forms, including ammonia (NH₃), ammonium (NH₄⁺), nitrite (NO₂^-) and nitrate (NO₃^-). Most plants take up nitrogen in the form of ammonium and nitrates, and although the preference varies according to species, ammonium is mainly preferred to nitrates. The math reason for this is that plants use ammonium to synthesize proteins, and if nitrogen is absorbed as nitrates, the plant must expend energy converting the nitrates back into ammonium. In the aquarium, ammonium is produced by fish in waste matter and as a result of the decomposition of organic materials. It is normally converted first into nitrites and then into nitrates by the bacteria in a biological filter. Many plants will take up ammonium before the filter bacteria are able to convert it although the two are both in competition for the ammonium. However, do not be tempted to reduce the biological filtration with the aim of increasing the amount of ammonium available to plants. In soft, acidic water, ammonium is not dangerous to fish but in hard water with a pH above 7, ammonium is converted into ammonia, which is highly toxic to both fish and plants, making biological filtration even more important in hardwater aquariums. Plants rely heavily on nitrates rather than ammonium as a source of nitrogen in hardwater aquariums. Although plants will often use nitrates only in quantity once the ammonium source is depleted, bear in mind that nitrates are a much safer source of nitrogen where fish are concerned, especially in harder water. Many liquid fertilizers contain nitrates as a nutrient ingredient, but it is important to keep a check on nitrate levels within the aquarium. In most cases, plants can obtain enough nitrogen from natural levels of nitrates produced as an end result of the biological filtration of aquarium waste (mainly from fish and, indirectly, fish food). Nitrate is easy to test for in the aquarium and many simple test kits are available for this purpose. Ideally, nitrate levels should be kept below 25 mg/liter. Many tropical aquarium fish can cope with levels higher than this, but in natural conditions plants rarely experience levels above 2 mg/liter, and levels above 30 mg/liter may be harmful.

Oxygen

Oxygen is taken up by plants in its gaseous form (O₂), as water (H₂O) and as carbon dioxide (CO₂). Oxygen is a vital structural component of cells and used during photosynthesis, although it is also a waste product of photosynthesis. Plants obtain the majority of their oxygen through their roots and from respiration. (It is also released from the roots.) Aquatic plants have large internal "channels" that make up a high proportion of their structure. These are used for transporting oxygen around the plant, most notably to the roots. Once oxygen is transported to, and released by, the roots, it combines with carbon and/or organic elements within the substrate, creating CO₂, which is taken up for photosynthesis. Releasing oxygen around the roots also helps to prevent localized anaerobic conditions, which can damage roots. Despite this high usage and waste of oxygen, plants do not do well in high-oxygen conditions and require only a small dissolved oxygen (D.O.) content. This is because when dissolved oxygen levels are high, a number of nutrients, especially iron (Fe), bind with oxygen and become too large to be assimilated by plants. High oxygen levels prevent plants from obtaining other vital nutrients in sufficient quantities. During the day, plants photosynthesize and produce waste oxygen, so there are never oxygen-deficiency problems at this time. The only time when oxygen becomes low is at night when plants do not photosynthesize but continue to use up oxygen through respiration. In a heavily planted aquarium with little water movement or a large number of floating plants, the air/water gas exchange is reduced and oxygen levels may drop severely. However, levels rarely drop too low for plants, although they may drop below the levels needed by fish. In most cases, it is not necessary to provide oxygen and/or aeration in planted aquariums.

Phosphorus

Phosphorus plays a vital role in energy transfer and is an important "ingredient" of genetic compounds and enzymes. Healthy root development and flower formation also depend on phosphorus availability within the plant. Phosphorus is taken up by plants through the roots in the form of phosphate (PO₄^-), which is present in the substrate at much higher levels than in the water. This is because phosphate will react with metal oxides - notably iron oxide - more frequently in open water, creating insoluble forms, such as iron phosphate, that cannot be used by plants. In open water, there is greater movement and mixing, hence the increased likelihood of contact between phosphates and metal oxides. This contact does not occur as often in the substrate, where phosphate remains in a usable form. In some cases, CO₂ produced by the roots during respiration can break down the bonds within insoluble phosphate compounds and allow phosphates to become available for plant uptake. Phosphate is often present in fish food, so levels are rarely deficient in the aquarium. In an average aquarium phosphate levels are frequently 1-3 mg/liter, while in natural conditions levels are normally only about 0.005-0.02 mg/liter. Low phosphate levels are not normally a concern, but high levels can encourage algae to bloom. To grow strongly, algae require phosphate levels above 0.03 mg/liter; since these levels are usually exceeded in the aquarium, algae blooms are often the result. Under normal conditions, most phosphate is "locked away" in the substrate and unavailable to algae. There should be no need to add phosphate to a planted aquarium, although it may be present in some nutrient-rich and soil-based substrates.

Potassium

Potassium is a very important plant nutrient that should not be ignored in a planted aquarium. It is a key component of a plant's biological systems and used in protein synthesis, the opening and closing of stomata (pores), seed development, root production, disease resistance, and photosynthesis. A potassium deficiency creates an overall weakness in a plant's development and appearance and also impedes photosynthesis. Plants absorb potassium as an ion (K⁺) from the water, rather than from the substrate, despite the fact that both in nature and in the aquarium, most potassium leaches from the soil or substrate. We do not entirely understand why this should be so, but allowing potassium to remain in the substrate may increase the availability of ammonium to plant roots. As tap water contains very small quantities of potassium, it is important to introduce it artificially to the aquarium, either by means of a liquid fertilizer or, more commonly, as one of the ingredients contained in nutrient-rich or soil-based substrates. Potash and/or granite dust are often mixed with nutrient-rich substrates to provide potassium.

Sulphur

Sulphur is used in the production of amino acids, proteins, and chlorophyll, and is normally present in adequate quantities in tap water. Plants absorb sulphur in the form of sulphate (SO₄^2-), and many aquatic soil-based substrates contain sulphate in quantities that are quite adequate for most plants. Some fertilizers also contain forms of sulphate. Rainwater also contains relatively high levels of sulphate, but these levels vary considerably. One of the main reasons why you should be cautious about using rainwater in the aquarium is the high level of sulphur that can be present during the first few minutes of rainfall. Sulphur in its raw form is a dangerous chemical and should not be introduced into the aquarium in large quantities.