Plant Filters For Home Aquaria
by Doug Dunlop, Calgary Aquarium Society
originally published in The Calquarium Volume 42, Number 7, March 2000
Although there is some debate over which organism is most responsible for removing the
waste products of fish from water in nature, there is little question that plants can and
do use many of these wastes from the water in which they grow. Many aquaria thrive and
have almost no nitrate in the water when fish loads are kept low and aquatic plants are
actively growing. All actively growing plants will remove some quantity of fish waste from
the water. Unfortunately, not all aquaria can be densely planted with aquatic plants,
either because the husbandry of the plants is difficult or because the fish are
incompatible with plants. In some specialized aquaria the situation is made worse by such
things as low pH that can slow or stop nitrifying bacteria.
Recently, efforts have been made in aquaculture to use the water from fish farming to
feed hydroponic crops in a process often called aquaponics. There has been a great deal of
success growing leafy greens, herbs, and other terrestrial plants hydroponically in the
water used for growing farmed fish, usually tilapia (Oreochromis sp.). This type
of system removes a great deal of nitrate from the water and allows the systems to be
recirculated many more times than had previously been possible. To put this into
perspective though, it is important to realize that farmed tilapia are raised at densities
equivalent to two 30cm oscars in a 40L aquarium!
Obviously, we neither want to nor should treat most of our fish the same way farmed
food fish are, but as with many industrial processes, there is often a way to scale things
down for use in the home. Plant filters can be an effective way to provide some or all of
the filtration to almost any aquarium.
There are several limiting factors in homes that need to be taken into account when
designing plant filters for the home. First among these is light. Most people have their
aquaria away from the sunny south window simply to avoid an excess of unattractive algae.
Unfortunately, this means that that the majority of fast growing food crops will not be
happy when grown hydroponically on the top of your fish tank. In fact, many plant filters
have been designed that use supplementary light in the form of fluorescent lighting to
encourage the plants to photosynthesize and thus remove more nutrients from the water. The
use of artificial lighting can provide other benefits as well. A reverse daylight schedule
can be used for the plant filter so that any oxygen provided by the plants is available to
the fish when they need it most, at night when any aquatic plants in the tank will be
Another limiting factor is nutrients. In typical hydroponic food culture there are
enough dissolved salts in the water to bring the conductivity to 2 to 3 mS/cm. This is
several orders of magnitude higher than the ideal for most fish. Hydroponic solutions are
also created by carefully mixing inorganic salts at specified ratios thus creating a
balanced solution for the plants. Many fish simply will not tolerate these chemicals. This
means once again that high-energy food crops are out of the question for most (if not all)
In getting around the limitations imposed on us, there are several options available.
The first is to provide supplemental nutrients. This approach is commonly taken in
commercial ventures but since this is an article about optimizing conditions for the fish,
I will not encourage it.
What I will encourage is the use of low energy terrestrial plants to strip the water of
Although almost any plant can be grown hydroponically, in the interest of convenience,
I prefer to use plants and systems that can be used over the long term with a minimum of
maintenance. I prefer tropical plants that spend most of the year actively growing and
tend to adapt well to the temperature of tropical aquaria. Further, the ability to survive
in relatively low light with low but constant levels of nutrients is a must. The plants
must have roots that will spread and cover a substantial area and they must thrive in wet
In the rainforest, I have observed several types of epiphytes and semi-epiphytes
growing in the tree canopy and up the trunks of trees. The plants that really struck me as
being ideal plant filters were the various types of philodendron which grow from the
forest floor and will wrap epiphytic roots around the trunks of trees as they make their
way up the tree toward the light. Watching the water run down the trunk of the trees (they
dont call it the rain forest for nothing!) I could see that the fine roots attached
to the trunk were ideally positioned to intercept any dissolved nutrients on their way to
the ground. Naturally, most of my plant filters since then have included some type of
philodendron. I have also had very good luck with Chinese evergreens and in one case Phalenopsis
orchids. When extremely messy fish are involved such as large cichlids, I have used faster
growing lettuce at the inlet to the filter followed by the slower growing plants closer to
In order to easily remove organic compounds from the water, plants make use of several
methods including ion exchange, usually with symbiotic bacteria (including our friends the
nitrifying bacteria) and often fungi which colonize the root system and break down complex
molecules into more easily assimilated forms. For this reason, plant filters remove the
highest percentage of nutrients from the passing water when slow rates of flow are used.
Higher rates of flow will mean that the plants will allow more nutrients to get passed
them. Slowing down the flow may be as simple as encouraging the plant to build a mass of
roots which effectively allows water to pass quickly around the mass while the center of
the root mass has the required slow flow. Water that flows through the roots of the plants
should be rich in oxygen in order to discourage root rot.
There are several methods of hydroponic and non-hydroponic culture that can be
effective for plant filters. The simplest is probably a Perlite bed fed by drip
irrigation. A container (usually less than 30cm deep) has a drain placed on the bottom
where it can drip back to the tank or sump. The drain is fitted with a screen to prevent
Perlite from flowing back to the aquarium. Plants are fed by slowly dripping water through
the Perlite bed. The flow rate should be low enough that the Perlite bed does not become
saturated. Since drip emitters can often clog when organic material passes through them, I
usually use a canister filter to pre-filter larger particles from the water. The plants in
this system will be drawing all of their nutrients from the water so it is sometimes
necessary to supplement with trace elements for the benefit of the plants. These plant
filters can be located quite far from the tank although they do need to be higher than the
water that they are draining to in order to avoid flooding. They also need occasionally to
have the drip rate adjusted or the drip hose cleaned. Peat or coir (coconut husk) can be
used instead of Perlite but the drip rate needs to be really slow and the tannins will
turn the water brown.
When using plants which root along the stem such as philodendrons, the plant itself can
be grown in a pot of standard potting mix while the stem roots are encouraged to grow into
the water. It is best to place the roots in the outflow from a power filter or in the flow
from an air stone to discourage root rot. The biggest advantage to this is that the plant
is not forced to gain all of its nutrients from the water and the potting mix can be
fertilized with trace elements that might not be desirable in the aquarium. This also has
an attractive "plant next to the aquarium" look to it that many people enjoy.
The fish seem to like the hiding places provided by the plant roots as well.
Plant filters can be an inexpensive and low-tech supplement to (or replacement for) an
existing filter. The plants are usually easier to care for than aquatic plants and in
cases where the use of aquatic plants is impossible, they are much better than no plants
at all. They are not a substitute for good aquarium maintenance but they can go a very
long way toward providing a better aquatic environment for your fish.
Other aquarium clubs and non-profit organizations can use this, or other articles,
in their own journals or web sites, provided that credit is given to the author, the
Calgary Aquarium Society, and The Calquarium. In the case of a reprint in a hardcopy
publication, two copies of the published work are sent to the Calgary Aquarium Society at
its mailing address. And in the case of a reprint in an Internet publication, a link back
to the original article site must be provided in a prominent location.