Aquarium Science Series 3: Plant Nutrients

Posted by Augusta Hosmer on

Read Time: 6 mins

Welcome back to part three of our Aquarium Science Series! This is a big one for planted tank enthusiasts: what nutrients do aquarium plants need and why? There are a whopping 17 essential nutrients for plant growth, but we'll make it easy.

Be on the lookout for a second half to this where we'll show you how and when to fertilize and supplement with these nutrients (and also the amazing way nutrients flow through plants).

Some Clarified Definitions

  • Nutrients- in botany, these are chemicals that helps a plant live, grow, and reproduce. Feel like this term gets muddled a lot with buzzwords in media, but it's a pretty simple definition.
  • Photosynthesis - the way a plant uses light to create its own "food" and energy. Uses water and CO2 to produce glucose and oxygen.
  • Respiration - What, you didn't know plants had lungs? Surprise: they don't. But they do go through a process called cellular respiration where the plant takes the sugars produced from photosynthesis and uses them to grow. The gist of what you need to know is it's just the mirror image of photosynthesis (uses oxygen and sugar and produces water and CO2).
  • Chlorophyll - green pigment in plants that absorbs light and makes photosynthesis go. Found inside chloroplasts (parts of a plant cell filled with tiny, disc-shaped structures) where photosynthesis happens. To put things in perspective, we're all alive right now because of some microscopic, sunbathing green CD's and a giant ball of fire. Do with that what you will.


A simplified drawing of the inside of a chloroplast
Above: a simplified illustration of a chloroplast

  • Metabolism - all the chemical reactions and changes keeping living things living. Several types.
  • Enzyme - a catalyst. Basically Adderall but for cells. Speeds up chemical reactions, metabolism, etc. Can break down or build up compounds depending on what's needed.
  • Chlorosis- scientific term for plant yellowing
  • Organic - involving carbon. Not referring to avocados that cost $16 and your spleen.
  • Carbohydrates - compounds like sugar, starch, etc. Used as "food" and energy by plants. Product of photosynthesis. Examples: glucose, sucrose, fructose (basically, you see a chemical ending in -ose, start thinking carbs)
Chart categorizing the 17 essential nutrients for plants

Elements Needed for All Life: Hydrogen (H), Carbon (C), and Oxygen (O)

These three elements are part of the foundation of life. They make up carbon dioxide (CO2), water (H2O), carbohydrates like glucose (C6H12O6) and countless other essential chemicals.

With aquarium plants, you'll never have to worry about hydrogen because they're literally swimming in it. They absorb hydrogen directly from the tank water. 

You do have to worry about providing enough carbon and oxygen in the forms of dissolved oxygen and CO2. Dissolved oxygen can be increased mostly with water surface agitation, but adding airstones and aquarium bubblers can help, too.

As for CO2, some plant species can go without supplementing, but those with higher carbon needs will thrive with either CO2 injection or a liquid CO2 fertilizer. Carbon is critical for plant growth and health and is a key component of chlorophyll. 

Macronutrients - The Big Three

The primary nutrients for plant growth are nitrogen, potassium, and phosphorus. These three elements are called macronutrients because plants need large amounts of them to grow and function. So if I'm ever having signs of nutrient deficiency in my planted aquariums, I usually start troubleshooting by looking at macronutrients.

Nitrogen (N) (the one you're probably most familiar with if you're into fish tanks)

Functions: name a function and it has a role. Huge piece of photosynthesis because chlorophyll is nitrogen-based. Building block of DNA, proteins, etc.
Sources: Aquatic plants can absorb nitrogen in a few forms. They can use ammonia and nitrates, but not nitrites. Ammonia is in fish waste (mostly from the gills, which surprises folks) and nitrates are in the byproducts of your beneficial bacteria breaking down nitrites. Decaying food and plants also can lead to nitrate/ammonia spikes. Fertilizers.
Signs of problems: lighter color; poor growth; shorter/more fragile stems; lower leaves browning/yellowing and turning transparent

Phosphorus (P)

Functions: component of DNA; energy metabolism/transfer; heavily used by soil microbes; photosynthesis/respiration

Sources: usually found in the form of phosphate, phosphorus can increase in your tank from fish food; tap water; certain kinds of rocks/soils; fertilizers; dead plants/dead algae

Signs of problems: darkening or bronzing on older leaves; thinner stems/shoots; delayed maturity/growth; smaller leaves; dark, soft patches on leaves

Potassium (K)

Functions: nutrient movement through plant; CO2 uptake; enzyme activation; maintains proper pressure within the plant; metabolism

Sources: some in fish food; fertilizers

Signs of problems: scorch; brown patches on leaves that turn into holes

Secondary Nutrients

Secondary nutrients are the middle of the nutrient scale, needed in smaller amounts than macronutrients, but in greater amounts than micronutrients. This category includes calcium (Ca), magnesium (Mg), and sulfur (S).

Calcium (Ca)

Functions: plant structural support; biochemical messenger

Sources: hard water; crushed coral; eggshells; cuttlebone; some from fish food; fertilizers

Signs of problems: poor/pale new growth; twisted and misshapen leaf growth patterns. Typically a problem with soft water

Crushed coral

Crushed coral, an easy way to supplement calcium

Magnesium (Mg)

Functions: component of chlorophyll (so heavily involved in photosynthesis); enzyme activation; phosphorus uptake

Sources: Hard water; fertilizers; Epsom salt

Signs of problems: one of the telltale signs is the rest of the leaf's losing color while the leaf veins stay dark. Typically a problem with soft water

Sulfur (S)

Functions: protein synthesis; chlorophyll production; used by beneficial bacteria in substrate; nitrogen fixation

Sources:  tap water; salts; fertilizers

Signs of problems: pale/yellow young leaves; slow growth; twiggy/spindly appearance (aquarium plants rarely have sulfur deficiency).

Micronutrients - Little But Fierce

Rounding out the list, plant micronutrients include zinc (Zn), iron (Fe), molybdenum (Mo), boron (B), copper (Cu), chlorine (Cl), manganese (Mn), and nickel (Ni). For some plant species, you'll also see cobalt (Co) and silicon (Si) listed as essential. Just depends. 

A liquid aquarium iron fertilizer in front of a planted aquarium

A liquid iron supplement for aquarium plants

These guys are needed in very small amounts, but don't let that fool you into thinking they're unimportant. Most of them function in enzyme reactions the plant can't survive without. This means if you have a deficiency in a micronutrient, it's important to correct, but not overcorrect. Iron, for instance, is a great supplement for red plants, but if you dose with too much, you'll end up with a headache of cloudy water, algae, etc. More does not always equal better. 

An important note with all of the nutrients we've talked about: none of them work by themselves. Nutrients work together within the plant, and altering one can alter another. 

Check out the first two of the aquarium science series, keep an eye out for more, and tank on! 

Aquarium Science Series Part 1: Water Chemistry without the Che-misery

Aquarium Science Series Part 2: Basic Aquarium Plant Botany

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