28.01.2007

Fishfood ≡ Aquarium Plant Fertilizer ?

To what extent can fish food be used as fertilizer for aquarium plants?

The use of a good fertilizer in planted freshwater aquarium is generally given a great importance. The relevant companies outbid each other in their promises regarding the expected plant growth on usage of their fertilizers. Large planted aquariums consume a large amount of fertilizer that becomes quickly pretty expensive. The leading companies are even offering different fertilizers, which are to be used together, usually an iron fertilizer, a general fertilizer, and a fertilizer with micronutrients. In the German aquarium scene the Internet distributed products by DRAK are regarded as a cheaper alternative. Many aquarists are reporting very good plant growth with this fertilizer and I agree.

The following picture, however, shows a detail from an aquarium with lush vegetation that is getting no additional fertilizer except fish food.

In this article I will discuss what contribute the added fish food makes to the fertilization of the plants in the aquarium, and whether an additional plant fertilizer is absolutely necessary in an aquarium where fish are kept.

Ingredients of fish food

Each fish food, whether it is live, dried or frozen food consists of living organisms or products derived from living organisms. All living things are built, despite their different appearance, chemically very similar and also have a similar need for nutrients. Thus, all nutrients that are needed by the aquatic plants can be found in fish food. For example consist animals, plants and fungi to 40 - 50% of carbon. From the 17 elements that plants need to grow only boron can not be found in animals. Also there are differences between animals and plants for example the content of nitrogen (N) and sulfur (S). Due to the higher demand animals have a approximately three times higher content of protein. But the question is are the quantities of the fish food ingredients sufficient to feed the plants in an aquarium ?

In order to answer this question, you have to know the ingredients of fish food and compare it with the needs of aquatic plants. The following table shows the amounts of the relevant nutrients in dry food [3] and, in comparison, the minimum concentrations of these nutrients in Elodea occidentalis [2]. According to Wentzel [1], the productivity of freshwater plants accounts 1.7 kilograms of dry weight per m² per year. Thereof the amount of dry food can be calculated, which would have to be fed each year to meet the needs of the plants.

Element	Average content in fish food (mg kg^-1)	Minimum content of nutrients of Elodea occidentalis (mg kg^-1)	Needed fish food(g)per year for a m² sized planted aquarium
B	48	1.3	46
C	430000	400000	1581
Ca	32000	2800	149
Cu	15	0.8	91
Fe	220	60	464
K	12000	8000	1133
Mg	1700	1000	1000
Mn	23	4	296
Mo	1.3	0.15	196
N	82000	16000	331
P	23000	1400	104
S	5100	800	267
Zn	110	8	124

Assuming that only 200 g of fish feed per month is given for an aquarium of 1 m² size, all nutrients should be sufficient so far. I think 200 g of fish feed per m² is not too much for a regular stocking. Downscaled to a standard 60x30 cm aquarium it would be only 36 g of fish food per month.The degradation of fish food

Fish food is metabolized in the aquarium by fish, snails and bacteria as in nature in plant available nutrients. Nitrogen (N), sulfur (S) and carbon (C) is metabolized to ammonium sulfate and carbon dioxide. Some nutrients are released directly from the fish in the urine or gills to the water, such as boron, potassium, magnesium, molybdenum, sulfur and carbon. Calcium, phosphorus, iron, copper, manganese and zinc are excreted in the feces and at least the metals are only available for plants with time for bacteria and fungi in duff. That's why the duff in the aquarium should not decreased to much to guarantee a continous metabolism of plant nutrients.

Even in basins with a very smal amount of fish the nutriens can be provided through fish food. Whether fish or bacteria are metabolizing the fish food is not really important. However, the amount of food should then be increased slowly to get the necessary amount of bacteria, so that the food does not rot. You can regard it as feeding the plant with nutrients. Fish and bacteria extract, beside of what they need for growing, mostly the energy and excrete the nutrients again. The plants are not addicted on the energy in fish food. They generate their energy with the help of light.

Carbon dioxide

A special case in this consideration is certainly the carbon. Even if the amount of fish food would be sufficient in theory, the resulting carbon dioxide is highly expelled over the water surface. Certainly carbon dioxide is the limiting factor at the sole fertilization with fish food. Therefore, it is not surprising that an additional carbon fertilization results often in an improved growth of the plants. If this is necessary in particular cases depends on many factors, such as the illuminance and the quantity of plants and fish.

Conclusion

Basically, you can assume that in an aquarium with fish stocking enough nutrients for healthy plant growth is brought into the water by fish food. An additional fertilization with plant fertilizer is therefore not catagorical necessary. Basically I would try to reduce the amount of fertilizer until the vegetation decreases. A huge amount of fertilizer often favors the growth of algae rather than the plant growth. Especially the iron content of the aquarium water seems to play a important role. A carbon fertilization, however, can lead to a significant increase in plant growth, because carbon dioxide is often the limiting factor for plant growth especially in bright aquaria.