Article: Adjusting Supplements in the Planted Aquarium

Carbon (C)

Bioavailable carbon (different from activated filter carbon) is vitally important to the health of a planted aquarium. Plants use about 3x as much carbon as they do any other nutrient, and a lack of carbon will cause slow or no growth, algae blooms, and poor utilization of nutrients. Most aquarists which experience issues with algae or poor growth in their aquariums can track these issues back to a lack of proper carbon supplementation. Here are two ways to supplement additional carbon:

Flourish Excel™

Flourish Excel™ is Seachem®’s liquid carbon supplement. Flourish Excel™ can be dosed daily to provide the plants with plenty of carbon, and will not alter the pH or any other water parameter.


In nature, CO2 is equilibrated through the water to feed the plants. Unfortunately, in the limited environment of the aquarium, this doesn’t happen quickly enough to meet the needs of the plants and they rapidly run out of CO2. However, it’s possible to artificially raise CO2 levels in the tank by using a CO2 injection system. The downside to using CO2 is that it inherently lowers the pH, and so it is necessary to closely monitor pH and alkalinity (KH) when using a CO2 injection system.

Macronutrients And Algae Control

Algae is a pest which grows whenever there are un-utilized macronutrients like nitrogen (N) or phosphorous (P) building up in the aquarium. Plants utilize these nutrients in order to grow and outcompete the algae, but if plants run out of any essential nutrients, they will not be able to outcompete the algae and an algae bloom will result. To prevent this from happening, it may be necessary to adjust macronutrients to match the needs of the aquarium. Here is how to adjust the three macronutrients:

Nitrogen (N)

Nitrogen is typically produced by fish in their waste (nitrate), although heavily-planted systems often require additional supplementation of nitrogen to ensure the plants have all the nitrogen they need. It is recommended to maintain nitrogen (nitrate) concentrations of about 10-20 mg/L for plant maintenance.

A tank which is lacking nitrogen will tend to have a buildup of phosphate and potassium and the plants will be yellowed. It’s also common to see hair algae in connection with nitrogen deficiencies, Seachem® recommends Flourish Nitrogen™ or aquavitro synthesis™ to solve this issue.

A planted aquarium with a buildup of nitrogen (nitrate) likely has a phosphorus (phosphate), potassium, or carbon deficiency.

Phosphorus (P)

Phosphorus can be produced by rotting organic matter and fish waste, although it is also common to get phosphate contamination in tap water. It is recommended to maintain phosphorous (phosphate) concentrations of about 0.5-1 mg/L for plant maintenance.

Phosphorus deficiencies will show similar symptoms to nitrate deficiencies: yellowing, necrosis, and a buildup of nitrate and potassium. It is also common to see green spot algae in connection with phosphorous deficiencies. Seachem® recommends Flourish Phosphorous™ or aquavitro activate™ to solve this issue.

A planted aquarium with a buildup of phosphorus likely has a nitrogen (nitrate), potassium, or carbon deficiency.

Potassium (K)

Potassium is often present in mineral supplements like Equilibrium™ or substrate, but unlike the other two macronutrients, an excess of potassium typically will not cause algae blooms. It is recommended to maintain potassium concentrations of about 5-10 mg/L for plant maintenance.

Potassium deficiencies typically result in “pinholes” in leaves as well as necrosis of older leaves. Like the two other macronutrients, if plants are lacking potassium, phosphate and nitrate will tend to build up in the aquarium, resulting in a variety of algae growth. Seachem® recommends Flourish Potassium™ to solve this issue.

A planted aquarium with a buildup of potassium likely has a nitrogen (nitrate), phosphorus (phosphate), or carbon deficiency.

Iron and Trace Elements

While carbon and macronutrient (NPK) deficiencies certainly show the most dramatic symptoms, there are several other nutrients which can also show distinct symptoms. 


Iron is an essential element, and is immobile in plants, which means that a plant cannot take iron from one part of itself and move it to a different part. The result of this is that iron deficiencies tend to appear as shriveled and twisted growth patterns in new leaves rather than older leaves. Seachem® recommends Flourish Iron™ to resolve this issue.

Trace Elements

Plants require a whole host of trace elements present in such low concentrations that it is almost impossible to test for them. When a plant lacks trace elements, it will tend to show a loss of color in between the veins as well as twisted or shriveled growth patterns. Seachem® recommends Flourish® or Flourish Trace™ to resolve this issue.

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