Nutrients can be considered as the plant equivalent of vitamins and minerals in animals. Plants use nutrients to control growth, repair, photosynthesis and a number of vital 'body' functions. Without a suitable supply of nutrients, plants may suffer from stunted growth, thin leaves, discolouration and general ill health. In the aquarium nutrients can be obtained from natural sources such as the waste matter from fishes and fish food, and many nutrients are present in normal tapwater. There are a wide range of different nutrients that plants require however, many of which are deficient in the aquarium. To provide the plants with all the nutrients they require, it is necessary to use artificial fertilisation, of which there are a number of methods.

How plants obtain nutrients
Plants use their roots to obtain most of their nutrients directly from the substrate. Although some nutrients can be absorbed directly from the water through the plants leaves, most nutrients are normally found in the substrate. Nutrients naturally 'bind' to organic matter, which collects in the substrate. Therefore the substrate acts as a nutrient 'sink' which concentrates nutrients and releases them at a steady rate. When fertilisers are added to the aquarium, the nutrients are held in the substrate for plants to obtain.

Oxygen levels and nutrients
Nutrients can also bind to oxygen molecules as well as organic material. When they bind to oxygen they become too large for plants to absorb and so become useless. High oxygen levels can cause nutrients already binded to organics to break their bonds and bind instead to oxygen. In a planted aquarium, oxygenation should be minimised to allow nutrients to bind to organic material rather than oxygen. Any air pumps should be removed and the outlet of filters should be placed well beneath the surface. Plants prefer a deep substrate because it provides low oxygen areas where nutrients can be released in a form that is easy for plants to absorb.

Liquid fertilisers
The use of liquid fertilisers is the easiest and most common form of fertilisation. Liquid fertilisers contain organic substances called chelates. Chelates bind to the nutrients and prevent bonding with oxygen molecules, unless the oxygen level is very high. Liquid fertilisers are an ideal choice for aquariums that may not have an ideal substrate because they do not need to be 'held' by the organics in a good planting substrate. Most liquid fertilisers need to be used regularly every week or every two weeks to maintain a constant level of nutrients.
There are a number of different types of liquid fertilisers available and there is a great deal of difference in quality. Some cheaper fertilisers may not contain a wide range of nutrients and can contain nitrates and phosphates, which may encourage algae. It is often best therefore, to spend a little extra and buy specific brands of liquid fertiliser. Ask a good retailer which brand they recommend or use themselves.

Substrate fertilisers
Nutrient rich substrate additives provide a constant release of nutrients into the substrate and last for many years. Once an aquarium is established, a good substrate will be continually replenished with nutrients from both fish and plant waste or from liquid fertilisers. The addition of a nutrient rich substrate provides a wealth of important iron and nutrient holding compounds, which help the substrate to act as a nutrient sink. Many substrate fertilisers are soil-like or based on clay compounds, which can only be added as part of the whole substrate when the aquarium is first set up. A heavily planted aquarium would benefit a great deal from the addition of a nutrient rich substrate additive.

Tablet fertilisers
Concentrated nutrients can be added to the substrate through the use of nutrient rich tablets. These are designed to be placed in the substrate beneath the roots of plants and are ideal for providing nutrients to individual plants. This can be beneficial in aquariums where only a few plants are used or where fast growing specimens require a greater quantity of nutrients. Nutrient tablets are also ideal for use when new plants are added to the aquarium.

Which method is best?
The method of fertilisation used depends on the number of plants in the aquarium and on the other environmental conditions. An aquarium with strong lighting, a good substrate and carbon dioxide fertilisation will contain plants that grow quickly and require a greater number of nutrients. An aquarium with only a few hardy plants will only need a small addition of nutrients, as the plants will obtain most of their nutrients from the water and natural organic waste. There are no set rules regarding the quantity of nutrients needed by an aquarium, so a little trial and error is required. It is important not to over fertilise, as this can encourage algae. Remember as well that nutrient supply is only one of the requirements of plants, so any problems with plant health or growth may not always be down to a lack of nutrients.

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