Problem solver: Poor plant growth


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Any problems associated with plant growth are usually easily sorted, as Jeremy Gay explains

In order to grow properly underwater, plants need access to light, carbon dioxide and nutrients — and conscientious hobbyists can provide all of these ingredients.


There are different artificial light sources available: from LED, T8, T5 to metal halide — and T5 further subdivides into linear T5 and power compact T5. All can be used to grow plants, though success is also down to the quantity, quality and duration of light.

Quantity refers to the intensity or concentration of sunlight. Quality refers to the colour or wavelength reaching the plant surface. Duration or photoperiod refers to the amount of time a plant is exposed to sunlight.

The quantity of light is difficult to recommend accurately and is tested professionally with a PAR meter. The home hobbyist is usually left with guidelines, like T8 being the least bright fluorescent by length and T5 is about twice as bright as T8 but differs from manufacturer to manufacturer.

Power compact offers a lot of light for its length, but is not as efficient at spreading light as a linear T5 in conjunction with a reflector. Metal halide will punch through deep water but isn’t necessary on any planted tank, unless about 91cm/36” deep.

If in doubt, go for T5 with reflectors. Aim for a size nearly as long as the tank, with two tubes being sufficient for most plant species. Four T5s with reflectors would be considered high light.

Another way is to use watts of light per litre of water: 0.5 watts per litre is fine for most plants and one watt per litre should be considered high light. However, LED, being low wattage and very high light, will eventually make that guideline redundant.  

Choose lighting with a colour temperature between 2,000 and 10,000K. It will usually state this on the box, and, if displayed as colours on a graph, you want a light that emits red, orange, yellow or green. Blue is too high a colour temperature, at about 14,000K plus, and will just be used by algae.

Light duration can also vary. Too little and your plants won’t grow, too long is more than the plants need and algae will use it instead. To understand what tropical plants need, look at their habitat.

Our plants originate in the hot, sunny areas of South America, Africa and the Far East.

A full day of sunshine for them may be 12-14 hours, so make 14 hours your absolute limit. But that sunshine isn’t always unbroken. It may rain, be overcast, or a tree may shade plants.

Generally 6-12 hours is fine. If you have an algae issue, cut lighting to six hours per day, but, if your plants are taking everything you throw at them and growing like weeds, give them 10-12 hours.

Carbon dioxide

This basic element will really help plants, as normal aquarium water contains less CO2 than the plants would ideally like. Carbon helps plants to build their structures and, by adding the gas to the water, your plants will grow better.

Some difficult plants like Glossostigma or Hemianthus callitrichoides 'Cuba' must have some form of carbon and bright light if they are to grow underwater. In the wild, carbon dioxide will usually be available to plants that break the water surface and take it from the atmosphere.

The downside of plants growing immerse in the aquarium is that they will drop all the underwater leaves that we find desirable and just concentrate on the bit poking through the surface instead. To encourage them otherwise we provide all the light, carbon dioxide and nutrients they need in the water.

Carbon is available in a number of forms and all can be effective. It can be introduced as a gas, either from a pressurised cylinder, or as a by-product of fermentation.

Generally, speaking, pressurised systems are the more expensive option but can be controlled to deliver a measured amount of gas into the water. They can be regulated, turned off and gas can be administered through different types of diffuser.

Fermentation sets are less expensive. Yeast and sugar are added to water where they produce CO2. This is then bubbled into the tank via a diffuser. This form of the gas cannot be accurately controlled or turned off, but is adequate for beginners. As it has a limit as to how many bubbles it can produce, and for how long, it is most effective on hardy plants that aren’t too reliant on high level CO2. Inexpensive aerosol gas cans are also available.

Liquid carbon is now available and this formula has proved to not only aid growth, but thought by some to also control algae. Dose daily and you don’t even need carbon dioxide in any other form, even for difficult plants.


Dosing of nutrients or plant foods is no longer just about dosing iron in a liquid form, and on a weekly basis. Many manufacturers are now producing foods that should be dosed daily so that the plants can take nutrients on demand instead of waiting a week each time.

Adding daily fertilisers, including trace elements like boron, copper, iron, manganese, molybdenum and zinc will aid your plants massively, and, if you don’t change water regularly, or use RO water, your tank could well be deficient.

And once you have a large biomass of healthy plants hungry for plant foods to aid rapid growth, you could even add nitrate, potassium and phosphate. However, you may cause algae if you overdose with nitrate and phosphate with insufficient plant biomass, so speak to an expert in plant fertilisation who can calculate a dosing regime for you — or buy all the traces and macros in one easy-to-dose bottle, off the shelf in the aquarium store.

A sure sign of nutrient deficiency in plants is the thinning and yellowing of leaves. Some experts even believe that plants showing pink hues in the leaves may be deficient, like Hygrophila polysperma (though there is also a pink variant available).

Other causes

Be very careful when selecting plants from aquatic shops, as some are not aquatic plants at all. If thick stemmed or variegated ask if they are aquatic and specify that you only want true aquatic plants. No amount of light, CO2 or nutrients will help you to succeed with what is basically a house plant.

Difficulty varies from species to species, and some are much easier to grow than others. If a new aquatic plant grower and want visible success, choose Vallisneria, Hygrophila, Green Cabomba, Amazon sword plants, Java fern (pictured above), Anubias and Bacopa. Difficult to grow species include Rotala, Red Myriophyllum, Glossostigma, Riccia and Hemianthus. To grow those you need high light, high CO2 and high nutrient levels.

Some methods of filtration are not ideal for growing plants. Undergravel filters create a lot of aerated flow through the gravel, and throughout the decades, plants have usually failed with the method. Air stones and Venturis drive off CO2 and oxidise nutrients, so will impair plant growth.

Lots of tropical and coldwater fish will eat the plants. You may actually catch them chewing on leaves, or find holes in leaves or floating leaves at the surface. Fish which dig excessively will also uproot and unsettle plants, and large fish can lift plants while looking for food. Ensure your fish selection is plant friendly.  

This article was first published in the August 2008 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.