Plants take up ammonium, which is toxic to fish. Yet they also protect fish by
removing heavy metals such as lead, cadmium, copper and zinc. But they also
extract nitrate. Now although levels of up to 50 parts won't have an adverse
effect on fish, higher levels are toxic. Besides, algae feed on nitrate, so
controlling this pollutant helps counter
One of the best plants to use for water purification, and which is sometimes
used in commercial water treatment plants, is the Water hyacinth. Yet most
floating plants work as water purifiers.
These include the Giant duckweed and Common hornwort, the latter which also
produces a natural algaecide to fight algae. This is a self-preservation measure
on the part of the hornwort, for if algae gets a hold, the hornwort would be
Floating plants have the advantage that they can remove CO2 from the atmosphere
without expending much energy. As they replicate with ease and great efficiency,
they also strip out nitrate at a rate of knots.
By covering the surface of water, these plants physically cut down the amount of
light entering the light. Again, this helps cut down on algae formation - and
gives fish shelter.
Most floating plants have long roots that trail downwards. These become colonized
with bacteria, acting as biological filters in their own right.
The pH of your tank will fluctuate quite dramatically during the course of day
and night as a number of processes take place in your tank.
First is the action caused by the friendly bacteria that oxidize waste from
fish. Known as the nitrogen cycle, these bacteria first reduce ammonia to
nitrite and then nitrate. This process of oxidization makes the water acid and
However, during photosynthesis, plants will remove CO2 from the water, making
the water alkaline and therefore neutralizing the effect of decomposition. So
plants help stabilize pH.
Photosynthesis also means that plants produce oxygen during the day, which
benefits the fish. These chemical process account for fluctuations in pH. As an
experiment, test your water in the day with lights on and at night with lights
off. You'll see what I mean.
There are two types of bacterial processes: aerobic and anaerobic.
Aerobic processes require oxygen. Such bacteria convert organic matter from fish
food, waste and other debris into nutrients that can be easily absorbed by
In soil where oxygen is not present, anaerobic processes produce iron and
magnesium. However, you also get that rotten egg smell that is produced by
hydrogen supplied, a gas that is extremely toxic.
Hydrogen supplied rises through the substrate and as it does, makes contact with
thousands of aerobic bacteria which oxidize out the harmful effect of the gas.
This bacteria receives the vital oxygen via the action of the plant roots.
Keeping it healthy
So really, the key to a healthy aquarium is good plant growth. Plants need
nutrients, and these are obtained from the water, a soil-based substrate, fish
waste and fish food. Most of these are provided through your weekly routines, so
savings can be made on fertilizers.
For a successful substrate, you need no more than 2.5cm/1" of loam-based
substrate - aquatic soil is ideal. Cover this with 5-8cm/2"-3" of fine
silver quartz gravel.
Encourage your plants to grow out of the water - they work more efficiently
growing in the air, which benefits the whole eco-system.
Don't be surprised when the shape of leaves growing out of water changes.
Classic examples are Hygrophilous corymbosa and difformis. Under water they
struggle to extract CO2 and the feathery leaves of difformis flop around,
gathering nutrients with the help of bacteria.
Out of water, the leaves and stems have to stand up to the elements, so the
stems harden and the leaves change from the narrow and elliptic to more oval.
The best thing is when they produce wonderful purple flowers that will flower
for more than four months.
The message is simple - let your plants breathe. Try a small, open-topped tank;
you won't be disappointed. When they grow out of the water, the effect is
Of course, plants need light. Daylight is ideal, and occasional sunshine is of
great benefit. Don't overdo it, though, as too much will cause algae problems.
If you can't achieve this, any good, cool white light works just as well.
This article was first published in the April 2005 issue of the Practical Fish keeping