This is a fairly common aquarium plant which is easy to find, but that can have very interesting characteristics, such as a wide adaptability to various aquarium conditions and the possibility of growing above ground.

Description of Hydrocotyle leucocephala

Hydrocotyle leucocephala is a plant of the Apiaceae family, the same family as carrots, caraway, parsnips and parsley.
Incidentally, if you try to chop up some Hydrocotyle leaves, the smell will be very pleasant and similar to that of parsley – but don’t use it in cooking!

The etymology of the name comes from hydor (water), kotyle (navel) and leuco-cephala (white head, referring to the inflorescence).

The English common name, on the other hand, is “Brazilian pennywort“.
From its English name, we can guess that it is a South American plant: in fact it is widespread from the south of Mexico to the north of Argentina.

Hydrocotyle is an amphibious, crawling plant that can live submerged, floating and partly emerged, in conditions of high humidity.
Thin, light green, rounded roots and leaves with diameters of up to 5-8 cm grow from each node.

In particular conditions, it can produce inflorescences on a peduncle about ten centimetres long.

Cultivation of Hydrocotyle leucocephala

Hydrocotyle leucocephala is very easy to find; practically every aquarium shop has at least a few specimens.
It is also possible to obtain new plants from the prunings of other enthusiasts: a small branch is enough to get you started.

In both cases, you are almost always starting with a submerged culture, so the plant will not have too many difficulties acclimatising.

Fertilisation

H. leucocephala is not fussy about nutrients.
Obviously it cannot grow without nutrients, but it will adapt its growth speed and leaf size according to the presence of nutrients.

In particular, if a lot of macro nutrients are present (especially nitrogen such as nitrate), it can take up a lot, especially if it is close to the surface or with emerged leaves.

I would also point out that the plant can be a very good indicator of magnesium deficiency: if the old leaves of the plant start to turn brown at the edges, it is highly likely that this element is missing.

It is also good to know that the plant, being very fast, can make other, slower plants deficient in nutrients (especially the aforementioned magnesium).

Carbon dioxide (CO₂) is not essential for its growth, especially if there are emerged leaves. However, a good presence of carbon can improve the growth of the submerged parts.

Finally, contrary to most other plants, the roots that Hydrocotyle emits from each node are not a sign of nutrient deficiency: they are always present.

The emerged part, if present, has no roots on the nodes.

Growth and propagation

Hydrocotyle leucocephala always tends to grow upwards, to reach the surface and grow floating, with emerged leaves, getting the maximum benefit in terms of exposure to light and carbon dioxide.

This can be prevented by cutting back any branches that tend to stick out of the water, forcing the plant to grow submerged.

However, if you have an open aquarium, Hydrocotyle can, in my opinion, make the most of its beauty with its emerged part.

To bring the plant to the surface, I have observed that pulling a branch of the plant out of the water rarely works.
Instead, I recommend waiting for the plant itself to decide to emerge.

If it does, the success of the emerged part is almost guaranteed (the only pitfalls are a too dry environment or proximity to lamps, which can burn the leaves with the heat produced).

Once emerged, the branches tend to lose their submerged leaves, as they become almost useless, being shaded by the emerged leaves and the latter having a much greater advantage in terms of light and carbon.

To avoid this, I recommend keeping some branches submerged by cutting them off before they emerge from the water. By doing this, these branches with leaves will hide the leafless branches that have emerged.

As far as propagation is concerned, nothing could be easier. Simply cut off a small branch, preferably submerged, keeping at least 3-4 nodes and you will have a new plant.

Location

Because of the way it grows, the ideal position is on the rear or on the sides of the tank.
In particular, if you have an open aquarium, with the lateral position it is easy to obtain a good development of the emerged part, as you can see in the photos above.

I would not recommend frontal positioning, as H. leucocephala will block the view of the aquarium, unless you grow it as a lawn (in which case you will need excellent light output, CO₂ and fertilisation).

For a lawn or frontal position, I would recommend the species Hydrocotyle tripartita, which has a more horizontal growth than H. leucocephala.

Conclusion

This is an extremely common plant, but if kept well (in an open aquarium, partly emerged!) it can give great satisfaction for very little expense.

It helps a lot with the absorption of waste substances and it is an excellent indicator for the deficiency of the nutrient magnesium (for which there are tests, but they are uncommon and relatively expensive).

It is also suitable for newbies, as long as even basic fertilisation is provided. As you become more familiar with the fertilisation and the plant, you can make it grow better and better.

Bibliography

Aquarium Plants, Christel Kasselmann, Krieger Publishing Company, Malabar, Florida, 2003.

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What does this mean?
Let’s see!

Exchange capacity

Substrates and soils contain net (i.e. non-equilibrium) electrical charges.
Depending on the type of resulting net charge – positive or negative – we will have different exchange capacities.

Cation exchange capacity (CSC – CEC)

Substrates containing particles known as aluminosilicates, i.e. crystals composed of aluminium (Al) and silicon (Si), or organic matter, have a predominantly negative electrical charge, which attracts positively charged particles, also known as cations.

There are basically two reasons for the presence of these negative charges:

1. breakage of the outer layers of the silicates that make them up, with exposure of the “edges”, which are usually negatively charged;
2. substitution within the structure of the silicates by certain cations, such as Mg²⁺, of other more charged cations, such as Al³⁺.

These negative charges, as we mentioned earlier, are able to attract positively charged ions (cations), such as the ammonium ion NH⁴⁺ or calcium and magnesium ions (Ca²⁺ and K⁺).

Cations are not attracted randomly: they are attracted with different strengths, depending on valence and other parameters (hydrated ionic radius etc) and those present in greater quantities are attracted more.

For example, some of the nutrients bind in this order:

H⁺ ≥ Al³⁺ > Ca²⁺ > Mg2+ > NH⁴⁺ = K⁺ > Na

H = hydrogen, Al = aluminium, Ca = calcium, Mg = magnesium, NH4+ = ammonium, K = potassium, Na = sodium

… where on the left are the most easily exchanged elements.
This series is a type of lyotropic series.

Similarly, some transition metals tend to follow this order:

Cu²⁺ > Ni²⁺ > Fe²⁺ > Mn²⁺

Cu = copper, Ni = nickel, Fe = iron, Mn = manganese

How is the exchange capacity measured?

The total amount of negative charge in a substrate is called the cation exchange capacity (CSC).

The CSC is commonly measured in milliequivalents [meq] per unit weight; one meq is equivalent to 6×10²⁰ (6 followed by 20 zeros) negative charges.

Below are some typical values for some substrates. To give a yardstick, in agriculture a soil with a CSC of more than 15-20 meq/100 grams is considered to have a good exchange capacity:

• sand 3-5 meq/100 grams
• clay soil 10-15 meq/100 grams
• silt 15-25 meq/100 grams
• clay 20-50 meq/100 grams
• zeolite 200-400 meq/100 grams
• humus 300-500 meq/100 grams (decreases with increasing pH)

To give an example in words, a substrate with a CSC of 10 meq/100 grams has 60 followed by twenty zeros negative charges per 100 grams!

Anion exchange capacity (CSA – AEC)

Contrary to what we have seen so far, different types of substrate, containing aluminium and silicon in amorphous (non-crystallised) form and iron or aluminium oxides, have a predominantly positive electrical charge, capable of attracting negatively charged particles, also known as anions.

Examples of anions are the sulphate ion (SO₄^2-), the nitrate ion (NO₃^–), the phosphate ion (PO₄^3-), the chloride ion (Cl^–) or the hydrogen carbonate ion (HCO₃^–, bicarbonate).

The behaviour is quite similar to that of cations, only the sign of the charge changes (negative, instead of positive).

Along the lines of the affinity order of cations, the binding order of some anions is as follows:

NO₃^– < Cl^– < SO₄^2- < PO₄^3-

Nitrates < Chlorides < Sulphates < Phosphates

i.e. nitrates are weakly bound, whereas phosphates are more strongly bound (and the exchange is therefore more burdensome).

The anion exchange capacity is always measured in meq/quantity of substrate or soil, similar to the CSC.

In reality, the anion exchange capacity is generally lower than the cation exchange capacity, so that only the latter is often mentioned. However, (at least) both are present and the anion exchange capacity is generally sufficient for the substrate to retain some anions, such as sulphates or phosphates.

Exchange capacity and aquarium plants

What is the point of all this discussion about exchange capacity?

When a substrate has a high exchange capacity it would be intuitive to think that it binds nutrients to itself and does not make them available to plants.

In reality, the forces of attraction between substrate and nutrients are relatively weak, allowing an exchange between the nutrients in the water, the nutrients on the surface of the substrate particles and the plant roots.

How a substrate facilitates nutrient uptake

When the roots of the plants move through the substrate, they either encounter the nutrient directly and take it up or they have to wait for it to arrive somehow.

A substrate with a good exchange capacity facilitates this work: the substrate has various nutrients loosely bound to it, which the plants can absorb and exchange more easily than fetching them from the water.

Taking up dissolved nutrient ions from the water, on the other hand, requires much more energy from the plants and some of them may not be able to do this at all.

How to manage a bottom with exchange capacity in an aquarium?

Many aquarium substrates have exchange capacity, such as fertile substrates with an organic or clay component or so-called allophanic substrates (a rather inappropriate term, as allophanic may be a component of these substrates but is practically never the only one), such as some ADA, Elos or Prodibio substrates or the common Akadama (bonsai soil), to name but a few.

Initial release and uptake

These bottoms tend to absorb nutrients as soon as they are placed in the aquarium, lowering measurable levels in the water.

It is common, for example, to observe significant decreases in carbonate hardness (absorption of carbonates), phosphates or iron.

Depending on the specific composition of the substrate, we may also see releases; iron and nitrate releases are not uncommon.
For example, ADA bottoms tend to release ammonium or nitrate initially; conversely, some batches of Akadama release iron.

The fact is that the substrate absorbs cations and anions from the water column and binds them to the particles in it.
If there was already some element being exchanged, it may be that by order of affinity this will be ‘displaced’ by other elements, so we have the releases measured above, in addition to the absorptions.

Fertilisation management

In the first period after set up, fertilisation must be carried out with particular care, both because the “active” bottom alters the expected values in the column as a result of its exchanges, and because the aquarium has just started, the plants have just been introduced and the various balances (chemical, biological, etc.) have yet to form.

It will therefore be necessary to take due account of what is being added and to observe the response of the plants, in addition to the test results (which can be misleading in these cases).

For example, many substrates tend to zero phosphates in the water: if we only looked at the tests, we would have to put a lot back in immediately.

However, phosphates are bound to the substrate and plants can take them up. Sometimes it can happen that the substrate releases, so if we have an increase in phosphate in an aquarium without having added it and we had previously measured an uptake, it is most likely that this phosphate is returned from the substrate.

Nutrient storage capability

It is also good to remember that these bottoms have a sort of nutrient storage capacity, but should not be bombarded with nutrients to saturate them quickly: the risk of sudden releases is high and risky. It is better to proceed with proper fertilisation over time, observing the plants and their possible signs of deficiencies or problems.

Finally, as far as durability is concerned, with time and the attainment of various balances the bottom will apparently stop exchanging, so we will no longer measure sharp drops in carbonates, phosphates or ammonium releases, just to name a few common phenomena.
This does not mean, however, that the soil has lost its exchange capacity, quite the contrary: a good balance between exchanged elements has been achieved.

This is different for so-called fertile soils, which lose their fertility over time as plants absorb the fertilisers they contain (iron, potassium, trace elements…). However, the component of these fertile grounds with exchange capacity – if present – retains its properties: if necessary, the fertiliser component can be replenished using sticks, tabs or fertilising spheres.

Credits

CSC diagram: By Kyle MoJo – Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=62069074

Allophane: By Rob Lavinsky, iRocks.com – CC-BY-SA-3.0, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15277038

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Easy-Life was founded in 1998 in the Netherlands and has since developed a unique separate element fertilisation protocol, which consists of a base fertiliser to be supplemented as required.

Let us now look at the components of this protocol.

Trace elements supplements

Easy-Life Profito

Profito is the basic product in the protocol and should be dosed regularly, adjusting slightly the doses according to the response of the plants.
It contains trace elements and chelated iron, magnesium and potassium.

Profito composition: 1% potassium oxide (= 0.83% potassium), 0.31% magnesium oxide (= 0.18% magnesium), 0.24% iron, 0.1% manganese, 0.02% boron, 0.003% copper, 0.003% aluminium, 0.003% cobalt, 0.003% nickel, 0.003% lithium, 0.003% vanadium, 0.002% zinc, 0.002% molybdenum, 0.002% selenium, 0.002% tellurium, 0.002% niobium, 0.001% scandium.
Chelating agents: EDTA, DTPA, NTA.

Dosage: 10 ml per 100 litres, once a week. It is advisable to increase or decrease the dosage by 40% according to plant response. It is also possible to dose 1/7 of the weekly dose daily.

Easy-Life Ferro

Ferro, as the name suggests (in Italian, Ferro means Iron), is a chelated iron supplement. It also contains a small percentage of potassium.
It should be used in cases where an iron deficiency is detected, for example when the leaves turn yellow (chlorosis).

Ferro composition: 1% iron chelated with DTPA, 0.35% potassium oxide (= 0.29% potassium).

Dosage: 5 ml per 100 litres, equivalent to 0.5 mg/l iron, every week until the deficiency is resolved.

Macro-nutrients supplements

Easy-Life Nitro

This is a nitrate and potassium supplement; it is in fact a solution of potassium nitrate.
Unlike other protocols, Nitro does not use ammonia or urea nitrogen, so the nitrate supply is fully and immediately detectable by a nitrate test.

Nitro composition: 17.5% potassium nitrate.

Dosage: 2 ml per 100 litres increase nitrate by 2 mg/l and potassium by 1.3 mg/l. Try to maintain a nitrate concentration around 10-20 mg/l.

Easy-Life Fosfo

Fosfo is a phosphate supplement. Use in case of slow growth or stunted plants.

Fosfo composition: 0.1% potassium phosphate.

Dosage: 2 ml per 100 litres increases phosphate by 0.1 mg/l. Try to maintain a phosphate concentration between 0.5 and 1 mg/l.

Easy-Life Kalium-Potassium

Kalium-Potassium supplements potassium. Use it if the potassium already introduced with Profito, Iron or Nitro is insufficient: white or discoloured leaves at the top of the plant or holes in the leaves are symptoms of potassium deficiency.

Kalium-Potassium composition: 10% potassium sulphate.

Dosage: 10 ml per 100 litres increases potassium by 4 mg/l. Use weekly until deficiency is resolved.

Other components

Easy-Life Root Sticks

Root Sticks are sticks of clay, naturally rich in iron, enriched with trace elements, to be buried in the substrate.
They are particularly suitable for root-feeding plants such as Cryptocoryne, Echinodorus or Nymphaea lotus. They can be broken up.

Root Sticks composition: not declared by manufacturer, but it is likely that the sticks contain iron and trace elements.

Dosage: apply one piece of stick every 10 cm approx. A stick may last several months.

Easy-Life EasyCarbo

EasyCarbo is a carbon supplement. It is a solution of glutaraldehyde, a carbon compound that is taken up by plants to synthesise the sugars they need.

Composition of EasyCarbo: glutaraldehyde solution.

Dosage: 1 to 4 ml per day per 100 litres, depending on the needs of the plants. Do not exceed the recommended dosage.

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