Feeding Aquarium Plants

Plants require a number of organic and mineral nutrients in order to maintain steady growth and good general health. Most of these nutrients are only require in tiny amounts but without them, vital biology functions cannot be carried out properly. Nutrients can be considered as a plant’s ‘diet’; without a proper diet, health problems arise and the plant will become ‘ill’. The number of nutrients that any given plant requires is extensive and can be provided in a number of ways. Looking at the function of various nutrients, their availability in the aquarium and assessing their importance is a good way of devising a ‘shopping list’ for suitable sources of fertilization.

Macro- and micronutrients
Plant nutrients are often described as macro- or micronutrients, depending on the quantities of a particular nutrient required by a plant.

Macronutrients are required in the greatest quantities; these include calcium, carbon, hydrogen, magnesium, nitrogen, oxygen, phosphorus, sulphur and potassium. Many macronutrients are readily available in the aquarium; for example, oxygen and hydrogen are normally always present in more than sufficient quantities, whilst calcium and nitrogen are usually present. Calcium is only found at low levels in very soft water and nitrogen can be absorbed by plants in the form of nitrates and ammonium, which are normally present as a result of biology filtration or from fish/organic waste. Therefore the only macronutrients that the aquarist normally needs to supply are carbon, magnesium, phosphorus, sulphur and potassium.

Micronutrients are only required in very small quantities and are often described as trace elements. Micronutrients include boron, copper, manganese, molybdenum, chlorine, nickel, iron and zinc. Both macro- and micronutrients are equally vital to the overall health of aquatic plants. Whereas macronutrients are usually used for structural components, such as cells, proteins and fats, micronutrients are used for cellular functions and the activation of vital enzymes. Micronutrients can be consider as important for the correct ‘management and control’ of plant biology. ‘Trace elements’ can be found in many liquid fertilizers, as well as in most tap water sources, but are often used up quickly in aquariums both by plants as nutrients and through binding to organic molecules.

Sources of nutrients
In the aquarium, nutrients can be supplied to plants from a number of sources. Because plants take up nutrients both through their leaves and the roots, nutrients should be made available in the substrate and the water. Micronutrients, or trace elements, are only needed in small amounts and are often present in most tap water supplies. However, some may quickly bond with other elements, making larger molecules that are unavailable to plants. They may also need to be supplied in quantity in the substrate or additionally through liquid fertilizers added to the water. The major difference between liquid fertilizers may need to be replaced weekly or every two weeks, whereas substrate fertilizers are normally present for longer periods. The tank substrate acts as a ‘storage’ facility for some nutrients. The lack of high oxygen levels and water movement in a compact substrate will prevent nutrients from being moved around, oxidized, bonded with carbonates or otherwise made unavailable to plants. In addition, the high amount of organic matter in most substrates allows natural chelates to bond with nutrients, allowing a large quantity of nutrients to be present, while a slow release reduces the amount of ‘available’ nutrients. Substrate fertilization can be achieved using propriety substrate mixes or tablet fertilizer.

Nutrient-rich substrates
Ready-made, nutrient-rich additives are widely available and usually designed as a main substrate, or to be mixed with small-grade inert substrate. These substrates are high in many of the nutrients required by plants and not commonly available through other sources (tap water, natural processes, etc.). In an established aquarium, most of these nutrients are released slowly over long periods of time, making nutrient-rich substrates an ideal long term fertilizing solution. Most nutrient-rich substrates will only begin to run out of nutrients after two to three years. However, if you carry out regular small water changes and allow some organic waste to build up in the substrate, it will naturally become a ‘sink’ for trapping and slowly releasing nutrients indefinitely. Regular small water changes and liquid iron fertilization should be enough to continually ‘recharge’ an established substrate that incorporates a suitable nutrient-rich additive.

Tablet fertilizer
Tablet fertilizers provide a localized supply of nutrients. They are concentrated forms of nutrient-rich substrate additives and particularly high in iron. Some faster-growing plants use vast amounts of iron, and supplying a concentrated source at the roots will help to prevent iron deficiency problems. Deficiencies in other plants, which may not be able to compete for available iron as quickly, will also be reduced or prevented. Do not use tablets as a ‘whole-aquarium’ solution to universal fertilization or iron fertilization, but only to provide an additional source of nutrients for individual plants. Tablets are not required, even locally, when soil-based substrates are used.

Liquid fertilizers
Several ‘off-the-shelf’ liquid fertilizers are available for aquatic plants, but they should be used with caution because over-fertilization can cause problems with algae and metal toxicity. Generally speaking, you get what you pay for when you buy liquid fertilizers; some of the more specialized products are far more valuable and contain the correct quantities of the required nutrients without oversupplying or lacking some elements.
Liquid fertilizers can be particularly useful in supplying chelated iron to the aquarium. Although iron is a micronutrient and only needed in small quantities, it is often unavailable in the aquarium unless it is provided in a chelated form that will slowly release a useable form of iron over long periods. Many of the nutrients in liquid fertilizers will become unusable after a short period, usually through binding with other elements or through oxidation. For this reason it is important to dose the aquarium on a regular basis, normally weekly or fortnightly.

Carbon dioxide fertilization
In most planted aquariums, CO2 fertilization is vital for plant health and is often the limiting factor in overall growth. Without adequate levels of CO2, plants cannot photosynthesis effectively and therefore cannot produce the energy needed to perform basic physiological functions. There are several ways of introducing CO2 into the aquarium. It is created naturally through fish and plant respiration, but mostly by bacteria as they break down organic matter. Many soil-based and established substrates continually release CO2, which can be used by aquatic plants. However, the quantities produced by these processes are minimal and would not be enough for heavily planted tanks. This is why additional fertilization is essential.
Furthermore, the air/water exchange in an aquarium continually releases a large quantity of CO2 into the atmosphere that must be replaced. Various devices designed to introduce CO2 into the aquarium are available for hobbyists and these include those using tablets that slowly release CO2, slow-release chemical reactors and pressured CO2 cylinder that can be adjusted and set by timers. All these systems introduce CO2 gas directly into the aquarium water. The aim is to keep the gas in contact with the water long enough for it to be available for plants to absorb.