The Aquatic Garden FAQ

Author: Dr. L.E. Freese

As every gardener knows, fertilization of the soil is essential for the growth of plants should this then not be a logical assumption for their aquatic counterparts?

 The common believe is that the plants in an aquarium have enough to feed on from the waste products of the fish and that too many nutrients are the cause of algae blooms. This is sadly not always the case, as the nutrients that are released often favour algae growth rather than plant growth.

Contrary to popular belief fertilizing your aquatic flora does not necessarily lead to an increase of algae, but can inhibit the growth of it. Studies have shown that if the nutrients in the water are in a ratio that the phosphate is present in a lower concentration than the other nutrients, the higher plants outcompete the algae for the phosphate and thus the algae concentration diminishes.

The nutrients that are required by the plants can be divided in macro- and micronutrients. The macronutrients (Nitrogen, Potassium and Phosphorous) are elements that the plant uses the most of, the micronutrients (Iron, Magnesium, Manganese, Boron, etc.) on the other hand are only utilised by the plants in small quantities. The micronutrients are present in tapwater, but there may be some that are deficient depending on the area where you live, therefore it is essential that they are added to the aquarium. Micronutrients need to be replaced frequently due to their unstable nature in water. Iron for example rusts (oxidizes) and in so doing becomes unavailable to the plants. The Iron that I present in fertilizers should be in a chelated form, as this does not oxidize and therefore is available to the plant for longer periods of time.

The proper levels of nutrients that need to be added to the aquarium varies depending on the fish and plant load, water chemistry, water change frequency and lighting. It is therefore best to follow the instructions of the fertilizer used and then experiment with the dosages if the desired results are not attained. A common mistake is to expect miracles; the plants will not react immediately, but will require a month or two for the change to be noticeable.

There are 2 methods of fertilizing your plants: substrate and liquid fertilization. The aquarium can be fertilized with tablets, powder or sticks that are added deep into the substrate and they slowly release their nutrients over a period of time. The advantage of this method is that the nutrients are available to the roots and the water column is not affected. Liquid fertilization needs to be added frequently (daily or weekly) and the advantage is that the micronutrients levels can be upheld and there are plants that do not have roots that enter the substrate and absorb their nutrients from the water.

In conclusion there are a few things to remember; firstly never add a fertilizer to the aquarium that does not state the components of it, as you may be adding something to the water that is harmful to fish. Secondly do not use garden fertilizer, as they contain urea which is essentially ammonia and this is deadly to fish. Garden fertilizer contains high levels of phosphates, which will cause an algae bloom and thus decrease the visual pleasure of a planted aquarium. Lastly certain micronutrients may accumulate in the aquarium and this could lead to complications with the fish in the water, by doing 25% water changes every 2 weeks this dilemma can be averted.  

November 2001