Fertilizing the Planted Aquarium

By Jeff Ucciardo. Edited by Kris Weinhold

Table of Contents

Types of Fertilizers
Driving Factors
Dosing Methodologies
Estimative Index (EI)
Perpetual Preservation System (PPS-Pro)
Jeff’s Regimen
Nutrient Deficiencies
Words of Advice
Additional Resources


Fertilizers are an integral part of keeping the aquatic plants in your aquarium healthy. All plants need to obtain nutrients from somewhere in order to survive. Sometimes this is from the substrate where rich soil, or supplemental fertilizers are present. This article will discuss the method of adding nutrients to the water column for plants to use.

Types of Fertilizers

Fertilizers are grouped into two types.

Macro (NPK):

  • Macro nutrients are the three nutrients plants need the most of to grow: nitrate, potassium, and phosphate.
  • We cannot dose macro elements directly, however, so we need to use these compounds, trying to achieve the following levels in our aquariums:
    • Potassium Nitrate – KNO3: NO3 range 5-30 ppm
    • Potassium Sulfate – K2SO4: K+ range 10-30 ppm
    • Mono-Potassium Phosphate – KH2PO4: PO4 range 1.0-2.0 or higher ppm


  • Also referred to as trace elements, micro nutrients are just as important as macro nutrients, but are sometimes over looked.
  • Without micro nutrients, plants cannot flourish, and will show signs of deficiency.
  • Common micro nutrients include Iron (Fe), Boron (B), Calcium (Ca), Magnesium (Mg), Cobalt (Co), Manganese (Mn), and many others.
  • To add these elements, there are several commercial products available:
    • Seachem Flourish, CSM+B, Tropica Plant Nutrition are well know examples.
  • Some important micro nutrient levels to aim for are:
    • Fe range 0.1-0.5ppm
    • Ca range 10-20 ppm
    • Mg range 2-5 ppm
    • Other trace materials don’t have a specific range

Driving Factors

The amount of fertilizers that you need to add to your aquarium is unique to every individual aquarium. There are several driving factors that must be considered to determine the proper level of dosing.


  • A high light tank requires more and constant fertilizers. Levels will drop quickly because the plants grow fast and robust with high light.
  • Low light tends to be easier to maintain and more forgiving because plants grow slower.
  • Light level guideline:
    • Low Light ( Under 2 wpg) 8-10 hours a day
    • Medium Light (2.0 – 3.0 wpg) 8-10 hours a day
    • High Light ( 3.0 – 4.0 wpg) 8-10 hours a day

Carbon Dioxide (CO2):

  • In addition to dosing fertilizers, plants also need CO2 added to the water. This may be even more critical to a successful planted aquarium than fertilizers.
  • Your goal is 15-30ppm of CO2.
    • Higher concentrations can kill your fish.
    • Lower concentrations can allow algae to outcompete your plants.

Plant Mass:

  • Higher plant mass requires more fertilizer; less plant mass requires less fertilizer.
  • On a new setup, pack the tank with fast growing plants, even if you don’t plan to keep them.
  • When you trim your plants, it is a good idea to factor in the difference in plant mass when you resume dosing.

Dosing Methodologies

Up to this point, fertilizing your aquarium might sound like a daunting challenge. Fortunately, several methodologies have been developed by hobbyists to deal with this problem in a more systematic way. This section will cover a few of the more popular methods.

Estimative Index (EI):

  • The idea behind EI is to simply flood the aquarium with an excess of nutrients.
  • The excess of fertilizers ensures that nutrients will always be available to plants.
  • The implication of adding extra fertilizers means that you need to do weekly water changes to rebalance the amount of nutrients in the water column.
  • Pros:
    • Measuring specific nutrient uptake rates is not necessary, therefore no test kits are involved.
    • Because nutrients are always available, helps to prevent plant deficiencies.
    • Plant growth is fast with the idea that the plants out compete algae growth.
    • Works especially well for high light/high CO2 setups.
      • Low light aquariums can use this method, but less fertilizers are needed.
  • Cons:
    • You use significantly more fertilizers than you need to. This adds cost, and means that you ought to consider the implications of where that nutrient rich water goes after you remove it from your tank.
    • 50% weekly water changes are required.
    • Fast growth makes it harder to maintain a scape.
  • Guidelines:
    • 10- 20 Gallon Aquariums
      • +/- 1/8 tsp KN03 3x a week
      • +/- 1/32 tsp KH2P04 3x a week
      • +/- 1/32 tsp (2ml) Trace Elements 3x a week-Opposite day of macro
      • 50% weekly water change
    • 20-40 Gallon Aquariums
      • +/- 1/4 tsp KN03 3x a week
      • +/- 1/16 tsp KH2P04 3x a week
      • +/- 1/16 tsp (5ml) Trace Elements 3x a week
      • 50% weekly water change
    • 40-60 Gallon Aquariums
      • +/- 1/2 tsp KN03 3x a week
      • +/- 1/8 tsp KH2P04 3x a week
      • +/- 1/8 (10ml) Trace Elements 3x a week
      • 50% weekly water change
    • 60 – 80 Gallon Aquariums
      • +/- 3/4 tsp KN03 3x a week
      • +/- 1/4 tsp KH2P04 3x a week
      • +/- 1/4 tsp (20ml) Trace Elements 3x a week
      • 50% weekly water change
    • 100 – 125 Gallon Aquarium
      • +/- 1 1/2 tsp KN03 3x a week
      • +/- 1/2 tsp KH2P04 3x a week
      • +/- 1/2 tsp (30ml) Trace Elements 3x a week
      • 50% weekly water change

      Note: K2SO4 is not required for dosing unless you need the extra Potassium (K). This K is found in KN03 and KH2P04. Dosing these two according to above will yield sufficient K levels. If you need to increase their K levels with K2S04, add the same measured amount as KH2P04. For example, if you are dosing 1/2 tsp of KH2P04, then dose 1/2 tsp of K2S04.

Perpetual Preservation System(PPS-Pro):

  • The goal of PPS-Pro is to grow healthy plants with minimal effort.
  • There is no testing required and water changes are optional.
  • Pros:
    • Attempts to provide slow healthy growth.
    • Few water changes required.
    • Aims to use only the fertilizers your plants need.
  • Cons:
    • Must premix your fertilizer solutions.
    • Requires a gram-scale to measure fertilizers.
  • Guidelines:
    • You must mix two solutions, a macro and micro solution:
      • Macro Solution:
        • In 1 liter bottle or 1000ml:
        • 59 grams K2SO4 (Potassium Sulfate)
        • 65 grams KNO3 (Potassium Nitrate)
        • 6 grams KH2PO4 (Mono Potassium Phosphate)
        • 41 grams MgSO4 (Magnesium Sulfate)
        • Fill with distilled water and shake well. Let sit overnight.
      • Micro Solution:
        • In 1 liter bottle or 1000ml:
        • 80 grams of CSM+B or equivalent trace element mix
        • Fill with distilled water and shake well. Let sit overnight.
    • Dose 1 ml of each solution per ten gallons of tank size.
    • Dose prior to lights turning on.
    • You will need an accurate scale
    • DO NOT mix the solutions together in one bottle.
    • Dose at separate ends of the tank or a few minutes apart.
    • The following is an estimate of what is dosed per day.
      • NO3 is dosed at 2 ppm per day
      • K is dosed at 2.7 ppm per day
      • PO4 is dosed at .2 ppm per day
      • MG is dosed at .2 ppm per day

Jeff’s Regimen:

  • Start with EI recommendations. But about half.
  • Test the water every other day for NO3 and PO4 to ensure they’re within proper levels; adjust as needed
  • Use plants as indicators to see whether they look healthy or deficient; adjust as needed.
  • Use algae as an indicator that something is unbalanced; adjust as needed.
  • Once a baseline for dosing is established, use that to project the required fertilizers for a 2-week period.
  • Setup an auto doser using that baseline.

Nutrient Deficiencies

While using test kits is a great way to determine what the current level of nutrients in your tank is, plants can also provide visual cues that something is wrong. Here are some common nutrient deficiencies along with what to look for.

  • Nitrogen (low): Leaves turn yellow, red leaves, (high) red plants are green
  • Potassium (low): Pinholes in leaves, ragged leaf edges and tips
  • Phosphates (low): green spot algae, stunting
  • Iron (low): Leaves grow in pale or yellow
  • Calcium: Distorted leaf growth, cupped or twisted leaves

Words of Advice

  • Take your time.
  • Make changes to one thing at a time. If you change too many variables at once you don’t know what it was that really improved things.
  • Allow a week or two to really see the effects of your changes.
  • Keep a notebook of what you are doing.

Additional Resources

Aquatic Plant Central Fertilator
Aquatic Plant Central – Fertilizing Forums
Rex Grigg – Online source for fertilizers.
Green Leaf Aquariums – Another source for fertilizers