Chemical methods for determining the health of aquarium plants.
Redfield ratio.
The Redfield ratio is the proportion between nitrogen [N] and phosphorus [P] in water. This is important for understanding the balance of the aquarium ecosystem and keeping it free of algae. Researchers have used this ratio to determine the impact of these two elements on the appearance of certain types of algae in water bodies. It seems that the highest chances of getting green filamentous algae in an aquarium occur with a relatively high amount of nitrogen (N > 5 mg/L) in relation to phosphates in the water. Or, conversely, a low amount.
The optimal nitrogen-phosphorus ratio, i.e., the Redfield ratio (RR), is approximately 1 : 16, with 1 : 10-20 being acceptable. Buddy converted the complex calculation of phosphorus and 
nitrogen from the Redfield ratio, based on exact amounts of nitrogen in nitrate, nitrite, ammonia, ammonium, and phosphorus in phosphates, into a more practical proportion for aquarists based on nitrate and phosphate NO3/PO, calling this proportion the Buddy ratio. Converting the Redfield ratio to the Buddy ratio is very simple: — Redfield = Buddy x 1.45. The ideal Buddy ratio is 23, with 15-30 being acceptable. Following the Redfield ratio will not help when nitrate concentrations exceed 10 mg/L and phosphates exceed 0.2 mg/L — algae will be present in the aquarium, although if plant growth is rapid, there will be very few of them. Buddy gives an example of a stable proportion of PO4 = 0.2 and NO3 = 5 mg/L in a wonderful champion aquarium from a Dutch competition, which was maintained for years.
In Takashi Amano’s aquariums, the phosphate level is kept at no more than 0.1 mg/L, and nitrates at no more than 1 mg/L. This corresponds exactly to the Redfield ratio! Higher phosphate concentrations, 0.2 mg/L, are also possible, but this does not completely exclude algae growth, and their absence is not as stable.
Trophic State proportion.
In addition to the Redfield ratio, there is a method for determining the health of natural water bodies based on the so-called Trophic State. This also uses the Nitrogen : Phosphorus ratio. If a water body has a low amount of nutrients, it is said to have a low Trophic level — meaning it is — Oligotrophic (low nutrient base). If nutrients are sufficient but not excessive — Mesotrophic (moderate amount), and water bodies with an excess of food — Eutrophic (too much).
It is believed that a nitrogen-to-phosphorus ratio (sum of NO3 + NH3) of 10 : 1 reduces algae growth, whereas adding nitrogen will stimulate their growth. A ratio between 10 : 1 
and 15 : 1 is considered transitional; and at a proportion greater than 15 : 1, the water body is considered to be deficient in phosphorus. Moreover, at this and higher ratios, an increase in the proportion of phosphorus enhances algae growth. In principle, this corresponds to what was said above. Phosphorus is considered the main limiting factor for algae growth; at the same time, a very low concentration of nitrogen in the water does not guarantee the same suppression of algae growth as a lack of phosphorus. Thus, it is believed that the influx of phosphorus into the water body should be restricted more than that of nitrogen. An invasion of blue-green algae stems from a very low amount of nitrogen in the water body, and higher nitrate concentrations are even better — other species of algae will suppress the blue-green ones, significantly improving the condition of the water body.
NO3 < 0.3 mg/L — Oligotrophic — the water body is generally clean, deep, and free from plants and algae blooms. Although beautiful, it lacks sufficient nutrients to support large fish populations. Nevertheless, Oligotrophic water bodies often develop a good food chain capable of supporting a desired population of large predatory fish.
NO3 = 0.5-1.5 mg/L — Eutrophic — the water body is rich in nutrients and can support a large biomass (many animals and plants). Algae typically grow in them or blooms frequently occur. They contain plenty of fish, but are prone to oxygen deficits. Hardy fish often live in them. Deprived of oxygen in late summer, the hypolimnions (bottom layer of the water body) of deeper Eutrophic water bodies limit the number of cold-water fish and cause the release of phosphorus from the sediment.
NO3 = 0.3-0.5 mg/L — Mesotrophic — occupies an intermediate position between the Oligotrophic and Eutrophic stages.
NO3 > 1.5 mg/L — Hypereutrophic — an excessively high amount of nutrients leads to complete overgrowth with algae.
Main factors influencing algae development.
How to grow plants, not algae. 
