What are the nutrient requirements of aquatic plants and what is the correct dosage?

There is little published information on the growth performance and fertiliser requirements of individual aquatic plant species (Kasselmann 1999). This is probably mainly due to the fact that research into plant nutrition is predominantly focussed on important cultivated plants.

Much more important, however, are the interactions of a mixed culture of aquatic plants (and microbes and animals) in the aquarium itself. These are complex systems with species competing for nutrients (and light). The supply of "macronutrients" such as nitrogen and phosphate via the fish food, the lighting, the carbon dioxide supply and the water quality of the changing water also play a role and cannot be predicted exactly.

Another important point is the availability of water tests of sufficient quality. Apart from test kits for iron and copper as well as the macronutrients nitrate and phosphate, hardly any aquarium manufacturer has any other test kits in its programme. In this respect, the aquarium keeper is not at a disadvantage if he does not use other products such as the highly recommended tests from Macherey-Nagel GmbH & Co. KG from Düren, for example, aquarists must rely on the "usual" tests available from pet shops and their observing eye.

Therefore, all dosage information (regardless of the manufacturer) should only be regarded as guidelines. To illustrate this, one of the test aquariums for the Fermendo fertiliser is briefly described below:

• Gross volume 1,600 litres, real volume approx. 1,400 litres

• Lighting 2 x 250 W HQI, lighting time 12 h daily, additional strong lateral sunlight in some areas

• Stocking: Rainbow fish, suckermouth loaches, a few armoured catfish, 85 Otocinclus, a few Hara hara. Rather low stocking overall.

• Heavily planted with dominant Vallisneria gigantea, Vallisneria spiralis, Crinum thaianum and Hygrophila polysperma, plus about 3/4 of the substrate with Cryptocoryne x willisii, Cryptocoryne becketii, Cryptocoryne wendtii (different varieties), Cryptocoryne parva, various Echinodorus, Anubias and Microsorium (on roots).

• Moderate supply of carbon dioxide (approx. 60 bubbles/minute, to prevent biogenic decalcification, especially by the Vallisneria), introduced via a 30 cm long flipper.

• Water values: GH 17, KH 9 (slightly fluctuating), pH 8.5, nitrate fluctuating on average between 25 and 40 mg/l, decreasing between water changes, phosphate 0.1 - 0.25 mg/l fluctuating.

• Weekly water change about 25 % with initial water 50 mg/l nitrate and more than 2 mg/l phosphate.

This aquarium was fertilised with a pure iron fertiliser until the start of the test phase and the iron value was kept at around 0.1 mg/l. After the start of fertilisation with Fermendo aquatic plant fertiliser, the iron values were measured daily in some cases, the growth performance of the aquatic plants was estimated and the dosage adjusted accordingly.

Under the given conditions, fertilisation was continuously increased to 5 times the average recommended dosage of Fermendo in daily doses to ensure a stable iron value of 0.1 - 0.25 mg/l permanently between water changes. The nitrate and phosphate levels drop between water changes, which indicates that the overall nutrient conditions are depleted. The growth rate of aquatic plants is more difficult to estimate (measure), but a rough estimate is that the growth rate has increased by around 30 % since the start of fertilisation. There are no symptoms of deficiency in the aquatic plants.

Overall, despite all the limitations of the measurement results, it is possible to obtain interpretations and estimates of the nutrient requirements of an aquarium using water tests available from pet shops:

1. The nitrate values should fall continuously between maintenance measures (water changes).

2. The phosphate values should also fall continuously between maintenance measures (water changes).

Both (falling) values indicate that the aquarium as a whole is in a nutrient-consuming state, i.e. that sufficient nutrients are being consumed by the activity of the aquatic plants and, to put it simply, that the material cycles are largely closed and functioning.

3. the iron value should ideally not fall below 0.1 mg/l (before the water change), slightly higher values after fertilisation are not harmful to the system.

Interestingly, under such conditions, even "quite high" values for nitrate (25-40 mg/l in the test tank) do not play any role in terms of algae growth; this tank is largely algae-free (of course, the fish that eat the algae must be taken into account). Even the high amount of light with some direct sunlight from the side is not able to cause visible algae growth.

References:

Kasselmann, C. (1999): Aquarienpflanzen. – 2. Auflage Stuttgart (Ulmer)