In my post about the nitrogen cycle I wrote that the nitrate is used by plants. Of course there are a lot of different elements in water that are used by plants. Those are divided into macro and micro nutrients (by amount of them required). The main macro nutrients that we are interested in are: CO2, NO3, PO4. Those are providing plants with carbon, nitrogen and phosphorus.The micro nutrients like Fe (iron), K (potassium) etc. are usually taken from the soil, Fe and K is usually added with some plants fertilizers. Lets go back to the main pack, C, N and P. People were doing a lot of research in the area of the actual ratio of those three in oceans and living things. Redfield started this research in 1934. What he found is that this ratio is actually quite constant in the oceans, whatever deep you go to. The ratio C:N:P was calculated to be 105:16:1. Other research has gone to find out how the algae blooms happen, and how to control this process. Here findings were very nice for aquarists, actually this whole thing can be controlled. What actually determines type of algae to grow is ratio between nitrogen and phosphorus in your tank. If this ratio is low, you will get blue-green algae, if it is high, you will get green algae.
Calculating amount of the nitrogen and phosphorus in the water can be somewhat tricky. By making few assumptions like the fact that ammonium and nitrite levels should be close to 0 and actually nitrate is the main nitrogen source, the whole calculation can be made very simple. See above the diagram prepared by Charles Buddendorf (check his blog here). Find out the level of PO4 and NO2 in your water by using some test kit (I use Sera). Check in the table above what is your ratio. Add some more nitrate or phosphate if necessary (I use Easy Life Nitrat and Phospho).
The main thing is that this works well in open nature, not in your aquarium. Generally from the table you could think that with 20 mq/l of nitrate you can have 2mg/l of phosphate and everything will be ok. Unfortunately it is not true. The issue is that in aquarium any excess amount of phosphate or nitrate will be used by algae, the table will only tell you if it is more blue-green or green type algae you will have. Generally plants should be able to use all the nitrate and phosphate that you have in your aquarium, when you do test for it you should see almost 0mg/l of both. The weekly water change is necessary to keep those levels down. For me the Redfiled ratio is only valuable to find out what should be the ratio of Nitrate and Phosphate in the fertilizers I add when plants use more that in produced in aquarium.
The main thing is that this works well in open nature, not in your aquarium. Generally from the table you could think that with 20 mq/l of nitrate you can have 2mg/l of phosphate and everything will be ok. Unfortunately it is not true. The issue is that in aquarium any excess amount of phosphate or nitrate will be used by algae, the table will only tell you if it is more blue-green or green type algae you will have. Generally plants should be able to use all the nitrate and phosphate that you have in your aquarium, when you do test for it you should see almost 0mg/l of both. The weekly water change is necessary to keep those levels down. For me the Redfiled ratio is only valuable to find out what should be the ratio of Nitrate and Phosphate in the fertilizers I add when plants use more that in produced in aquarium.
The chemistry of your aquarium looks complex but by controlling only few elements you can have a nice setup. Make sure that you do your weekly 25-30% water changes and keep your Nitrate and Phosphate levels low. Use the Redfield ratio to dose additional N and P if your plants are needing it.
Cheers,
Marcin
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