Plant Nutrients and Fertilizers

All aquatic plants need nutrients. They require the structural elements of carbon, hydrogen, and oxygen, and plants gain those elements from water and CO2. Plants are also able to receive nutrition from nitrate compounds in the water column and substrate, products of the nitrogen cycle that breaks down waste products from aquarium inhabitants, including plant detritus. However, additional aquatic plant nutrients and fertilizers are essential for fostering healthy growth, vibrant coloration, and dense foliage in aquariums. By providing key macronutrients (N, P, K) and micronutrients (iron, magnesium), fertilizers prevent stunted growth and deficiency symptoms like yellowing leaves. They further promote ecosystem balance by enabling plants to out compete algae for resources.

Substrate Nutrients

  • Aqua soil, like Fluval Stratum, is advantageous, supplying basic nutrients a lot like rich soil in a terrarium
  • Sand and gravel do not provide nutrients, however over time waste from aquatic plants and animals can build a nutrient base
  • Nutrients in substrate via fish, snail, shrimp waste (poop/detritus)
    • Contains every macro and micro nutrient and mineral found in commercial fertilizers like root tabs
    • Inadvisable to rely solely on fish waste, even if you have a fish heavy ecosystem, so supplement fish waste with root tabs over time
  • Cation exchange capacity (CEC)
    • CEC is a measurement of a substrate's ability to soak up nutrients from the water column, like a sponge, making those nutrients available for root feeding plants
      • These stored nutrients are positively charged ions (cations) of elements like calcium, magnesium, and potassium
      • The substrate acts as a nutrient storehouse
      • Higher CEC values indicate better nutrient retention and soil fertility
    • Gravel's CEC = 1.3 (low absorption rate)
    • Volcanic basalt (eg., Eco-Complete) CEC = 15
    • Aqua soils (eg., Fluval Stratum) CEC = 45 to 50 (high absorption rate)

Root Tabs

  • Over time aqua soil loses nutrient potency (a year on average), despite the contributions of plant and animal waste and CEC
    • Stem plants can draw a lot of nutrients from the soil
    • Plants in high light environments with high photosynthesis draw more nutrients than lower light plants
    • Liquid fertilizer and animal and plant waste prolong the nutrient supply in aqua soil via CEC, but usually not enough for fast growing stem plants
  • Root tabs are a good way to replenish nutrients in the substrate
  • Root tabs can be spread around plant roots every 6 months or so

Liquid Fertilizers

  • Liquid fertilizer is an excellent supplement for aquarium plant nutrients
  • Most plants draw nutrients from both their roots and the water column
  • Liquid fertilizer not only prolongs life of the substrate nutrients, but also promotes plant health in general
  • Epiphytes depend entirely on nutrients in the water column because they are not planted in the substrate
  • Liquid fertilizers have two basic ingredient groups
    • Macro nutrients
      • Those that plants need in large quantities
      • These are the building blocks of plants, needed for physical growth (along with carbon from CO2)
      • Important macro nutrients
        • Nitrogen
        • Phosphorus
        • Potassium
        • Sulphur
        • Calcium
        • Magnesium
      • Plants also need oxygen and carbon, but they get those from the water and CO2, so not needed in fertilizer
    • Micro nutrients
      • Those that plants need in small quantities
      • These facilitate chemical pathways for the internal processes of plants
      • Important micro nutrients
        • Iron
        • Boron
        • Manganese
        • Zinc
        • Copper
        • Molybdenum
  • Liquid fertilizers come as all-in-one general application, or you can buy individual fertilizers for dosing individual elements separately
  • Testing for liquid fertilizer levels in the water column is best determined by measuring for nitrate, which is considered an indicator chemical, (if nitrate is high or low, then the other elements are either high or low accordingly)
  • Liebig's Law of the Minimum Factor, developed by Justus von Liebig
    • An organism's growth is limited by the single essential resource (nutrient, light, CO2, water) that is in shortest supply, not by the total availability of all resources
    • Eg., if a plant needs nitrogen, phosphorus, and potassium for growth, but phosphorus is scarce, adding more nitrogen or potassium won't boost growth until the phosphorus deficiency is addressed
    • For a healthy aquascape, Liebig's Law underpins fertilizer management, showing that balancing all essential nutrients is crucial
    • So, how then do we best dose liquid fertilizer in the aquarium water column?
  • Three main methods for dosing liquid fertilizers
    • 1) Estimated index, called EI dosing
      • Provide plants with more nutrients than they need, dosing a couple times per week, ensuring that plenty of each essential nutrient is provided, then we can just fine tune lighting and CO2 amount to balance the tank
      • Over the week the nutrient levels build up, then you do a large water change (50%) every week to restore balance and start over
      • Good for tanks with a lot of fast growing plants under high light and CO2 conditions
      • Downsides
        • A lot of trimming and maintenance for all those fast growing plants
        • High nitrate levels are realized which can be bad for sensitive fish
        • Red plants do not benefit because red colors are only prominent when nitrates are low
        • Frequent large water changes can become tiresome
    • 2) Lean Dosing (or lean fertilization)
      • Set lighting and CO2 at best guess amount, then fine tune the nutrient supply in order to balance the tank
      • Provide plants only what they actually need
      • Good for lower light, slower growing plants, and tanks with fewer plants
      • Good for fish sensitive to high nitrate levels and for bringing out the red colors in red plants
      • Good for minimizing trimming and maintenance
      • To determine the dose quantity, test the fertilizer level by measuring for nitrate (20 to 40 ppm is good)
        • If nitrate is high, decrease dose amount
        • If nitrate is low, increase dose amount
      • Downside to lean dosing
        • If you dose too little over time, plants can suffer, not right away but slowly, bit by bit
        • Sometimes by the time adverse effects manifest themselves it's too late to reverse the damage and the plant can even be lost
        • This is not such a problem for fast growing stem plants, but it can be a problem for slow growing epiphytes
    • 3) Third option for dosing liquid fertilizers
      • Middle of the road approach
      • Don't provide excess fertilizer, and avoid going too lean
      • Manage the nutrient levels via the nitrate test (20 to 40 ppm), and weekly 25% water changes as part of normal maintenance routine

Recommended Nutrient Levels for Plants

  • Varying opinions exist with wide range of parameters due to different types of aquatic ecosystems
  • Low light, low tech tanks with slower growing plants will have lesser needs
  • High light, high tech tanks with faster growing plants will have greater needs
  • Because each tank has its own needs, the "best" nutrient levels must be determined for each tank individually
    • Certain general guidelines can be followed as a starting point based on the type of aquarium
    • Ultimately each tank's nutrient levels need to be fine tuned with observation and some trial and error (experimentation), changing only one variable at a time

Numerous Liquid Fertilizers are Available, Which are Best?

  • Compare the macro and micro nutrients offered in each fertilizer
  • Look for a good balance that seems to address the needs of a particular kind of tank, the plant mass and bioload
  • If you have a lot of stem plants, high nitrogen (high nitrate) may be best
  • If you have a lot of slower growing or lower light level plants, then low nitrogen (low nitrate) may be best

Scheduling Fertilizer Doses

  • Small doses every day or every other day is best
  • From recommended dosage on fertilizer package determine how much is needed for a week, divide by 7
  • Plants are better at uptaking nutrients than algae, so if they get a little each day, the plants will out compete the algae for nutrients
  • On the other hand, with one large dose per week, there will be more nutrients in the water column than the plants can use at once, so it becomes available for algae
  • Consistency is key

For new tanks ease into using fertilizers to give system time to adjust and balance

General Starting Point Recommendations

  • If nitrate is low (10 to 20 ppm) add a dose of 2HR Aquarist APT3 to get nitrate closer to 30 to 40 ppm
  • Note that 2HR Aquarist APT does not contain calcium, may need calcium supplement like Seachem Equilibrium depending on the calcium/magnesium level of your tap water
  • If GH is 6 degrees or less, add some Seachem Equilibrium (also for pogostemon stellatus curling leaves--a calcium deficiency), note that snails and shrimp also prefer some calcium for shells
  • 2HR Aquarist APT3 used regularly, with Seachem Equilibrium used as needed for raising GH, baking soda for raising KH
  • Remember to ramp up dosage amounts in small increments and assess results before increasing dosage

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