When aquatic plants are growing well, they will consume the nutrients that are
available in the water. These nutrients fall into two categories: Macronutrients and
Micronutrients. Macronutrients are those which the plants needs in large amounts. Micronutrients are
needed in very small amounts. Micronutrients are also known as trace elements.
A quick side-note about requirements for plants: In addition to the macronutrients and micronutrients, plants also need CO2 and
light in order to grow. In fact, CO2 and light are MORE important than any
of the nutrients. Plants that do not receive enough light will just turn brown
and die. No amount of fertilizer will help in that situation. But most
beginners who add live plants to their tanks will attempt to improve the
plant growth by adding some fertilizers, since it's much easier to add
some liquid from a bottle than it is to add more lighting to an aquarium. Don't
repeat this mistake. The only thing you will grow is more algae!
Now back to the fertilizers.
Macronutrients and Micronutrients
The Macronutrients consist primarily of Nitrogen, Phosphorus, and Potassium. These nutrients are all
needed for plants to grow. In an inhabited aquarium, fish food and fish waste will provide some of
each of these.
As you hopefully know, fish produce ammonia as waste. And as you hopefully know,
bio-filter bacteria in the aquarium convert ammonia into nitrite, and other
bio-filter bacteria convert the nitrite into nitrate. All three of these (
ammonia, nitrite and nitrate) are forms of nitrogen, and all can be used by plants.
In most non-planted or lightly planted tanks, the nitrate level will slowly rise,
and the aquarist must do water changes to lower the nitrate level. Nitrate levels
over 20ppm can be harmful to fish.
In the case of a heavily planted tank, with lots of fast growing plants,
it's possible for the plants to completely consume all the nitrogen
produced in the tank. In that case, the addition of nitrate is needed to
keep the plants growing happily.
In a well lit tank, excess phosphorus (phosphates) can lead to serious algae problems, so extra
phosphate is almost never added. And most fish foods contain sizable amounts of phosphorus,
so the plants will most likely be able to get as much as they need.
Potassium is an important macronutrient, and it is commonly in short supply in an
aquarium. The amount of potassium from food and waste is often much less than the
amounts of nitrogen or phosphorus. So adding potassium is often a good idea.
There are several commercial aquarium plant additives that contain potassium. You can also
obtain Potassium Sulfate (K2SO4) or Potassium Chloride (KCl) from a gardening store as a source.
K2SO4 is often referred to as "Sulfate of Potash", and KCl is referred to as Muriate of Potash.
I only recently (in the past couple months) starting adding potassium, and the improvement in
plant growth and health has been amazing.
There are main micronutrients. They include Boron, Calcium, Chloride, Copper, Iron,
Magnesium, Manganese, Molybdenum, Sulfur, Zinc. These nutrients are often found in
small amounts in tap water, and in low growth conditions, it isn't necessary to suppliment
them. But with improved plant growth, these nutrients will be quickly depleted from
the water, and plants will suffer. For the Micronutrients, there are many commercially
available fertilziers. My favorite of them is Tropica Master Grow (TMG). It contains
iron, manganese, magnesium, sulfur, boron, zinc, and molybdenum (spelling?). Others are
available from Seachem (Flourish), and Dupla (Dupla drops). Alternatively, many people make
their own. This is known as PMDD (Poor mans dosing drops). The starting point for PMDD is a
trace element mixture. I use one called Microplex. Another common one is Plantex CSM. These
are often available from Hygroponics suppliers. More info on PMDD is available at:
Regardless of what you use for micronutrients, these are typically added every few days. This is
important because the iron and other elements will not remain available in the tank water for more
than a few days. Dosing of micronutrients is commonly done based on the Iron level. The
mixtures are all created so that by adding the right level of Iron, then the other elements will
be present in the proper amounts.
Forms of Fertlizer
There are two commonly used forms of fertilizer for the aquarium: Substrate fertlizers and
liquid fertilziers. The substrate fertlizers are those which are inserted in some
solid form into the substrate. The liquid fertilizers are added directly into the tank water.
One of the benefits of substrate fertilizers over liquid fertilizers is that
when properly used, the substrate fertilizers are only available to plant roots. Since
algae doesn't have roots, it can't get to the nutrients buried in the substrate.
Substrate fertilizers come in many different forms:
- There is a powdered substrate additive
known as laterite that can be mixed with the lower level of gravel to provide a source of
iron that plant roots can get to. It's important not to use too much laterite, and not to
use it in the upper layer of the substrate, or it will leak into the water, and cause VERY high
iron levels, which will lead to algae problems.
- Another common substrate fertilizer comes in tablets or sticks. These "plant tabs" or sticks are
sold specifically for aquarium plants, "PlanTabbs", Seachem's Flourish Tabs, and Tetra's Hilena
Initial Sticks. These are normally intended to be placed every couple of inches through out the
substrate. Some of these tabs provide iron, some provide macro nutrients.
- The last common substrate fertilizer is a "plant spike". These are actually sold for normal household plants,
but certain varieties of them are especially useful for aquatic plants.
These are sold in the gardening section of many stores. The most commonly used ones are "Jobe's Plant Food
Spikes for Lush Ferns and Palms". This variety has a very low phosphate level, which is important, since
an excess of phosphate will quickly lead to algae problems. The Fern and Palm sticks also contain very little
urea, a toxic form of nitrogen. Other varieties of the plant spikes contain much more urea, to the point
where enough might enter the water and could harm fish. These are solid "spikes" that get pushed into the
substrate where their nutrients are available to plant roots, but not to algae in the water. These plant spikes
provide macro-nutrients only. When using these, they should be used sparingly, only at the base of heavy root
feeding plants. Do not insert these over the entire substrate.
Liquid fertilizers are often an important source of nutrients for aquatic plants. Many aquatic plants
have specially developed to be able to efficiently consume nutrients from the water. Some plants don't
grow roots down into the substrate, and so their only source for nutrients is from the water.
Most liquid fertilizers are intended to be added frequently, normally daily or weekly.
There are a variety of different liquid fertilizers available for a planted aquarium. Most of the commercially
available liquids contain micronutrients. Some liquid fertilizers are available for adding potassium. I am not aware
of any commercial liquid fertilizer that contains nitrates or phosphates. This is because those nutrients must be
more carefully controlled to avoid algae problems.
When using a DIY form of fertilizer, like PMDD, or when adding potassium or nitrogen from
a non-commercial source, the dry ingredients are usually prepared into a liquid form. Dry
powdered chemicals are mixed in measured quantities with a known amount of water. Then a small amount
can be added as needed.
Some people have asked about using common household fertilizers like miracle-gro in their
planted aquarium. This is a bad idea, since most terrestial plant fertilziers contain high levels of phosphates.
In additon, many of them contain their nitrogen in the form of urea, which is essentially ammonia. And in a tank
that contains fish, urea or ammonia is toxic. I've tested miracle-grow in an uninhabited 10g tank, and adding just 5ml of
the liquid to the 10g resulting in ammonia levels off the chart for my ammonia test. The same hold true for many
hydroponics fertilziers. I would suggest that you never add any fertilziers to your tank unless you are sure you know what it contains.