The Open Aquarium
by Claus Christensen
Claus is Managing Director of Tropica Aquatic Plants, of Denmark
The open aquarium, without the top glass and with the lamps hanging from above,
provides a new dimension of the traditional aquarium and it allows plants to produce an
emergent inflorescence. Does it cause humidity problems in our homes? Will the leaves of
emergent plants dry out? What will happen to the fish?
The open aquarium is my favourite aquarium! It provides new dimensions for the aquarium
and the living room. We can observe the leaves of floating plants, flowering aquarium
plants such as Nymphaea (water lilies), and a lot of other aquarium plants that
are amphibious and grow out of the water providing great ornamental effects. In addition,
opening the aquarium offers more possibilities for exciting illumination.
But this beautiful aquarium may affect the indoor climate much more than a traditional
Relative air humidity
A frightening example was an aquarist with an open aquarium of 800 litres in a room of 16
m². He had discus in the aquarium and the water temperature was 28°C (82°F). Problems
cropped up because of the large aquarium and the huge evaporation caused by the relatively
great difference in temperature between the room and the aquarium. The wallpaper became
mouldy and the windows were constantly misty.
But usually open aquaria do not cause problems with damp if the following are observed:
- The surface of the aquarium should not exceed 2% of the room area. Thus, in a room of
40m² the aquarium should not exceed 500 litres.
- The difference in temperature between water and air should not exceed 5°C (9°F), to
keep evaporation at a minimum. We have seen problems in unheated rooms during the winter
because evaporation increases tremendously with the difference in temperatures.
- Make sure that the room is frequently aired - this is advisable anyway and it ensures a
healthy indoor climate.
- Finally, floating plants or plants with floating leaves will help to lessen evaporation
because they create a boundary layer of relatively stagnant air!
If these precautions are taken, the relative humidity will increase only by about 5%.
I have asked scientists working with health and air humidity, and recent research has
shown that people cannot feel variations of 30-70% in relative humidity. When the air is
recognized as dry or moist within this range, it is caused by other factors such as high
temperature, dust, or gases released from materials in the room, e.g. building materials,
carpets, paint, etc. Thus, people usually do not feel the increase in relative humidity
that an open aquarium causes. For example, one person emits 2.5 to 3 litres of water per
day (breathing, sweat, showering, cooking), so if an open aquarium of 800 to 1000 litres
evaporates some 15 to 20 litres per week, it simply amounts to an extra person living in
Some allergists are very sensitive to microorganisms and the substances they may emit.
But recent research has shown that growth of microorganisms is independent of the actual
relative humidity. Microorganisms rely on the microclimate, which is created by the
fabrics on which they live and not the actual relative humidity as such. Aquaria in
general - traditional as well as open - have been examined in relation to allergy, and a
few cases have shown some negative effects, but not due to increased relative humidity.
They have all been caused indirectly by problems created by leaky tanks, which caused for
example growth of moulds in the carpet. In really rare cases people have developed allergy
to red midgets (chironomids), an allergy caused by the red protein haemoglobin.
From the research on indoor climate it may be safe to conclude that the effect of open
aquaria may be purely positive - a relaxing effect, psychological in nature.
Different types of lamps can be used for the open aquarium. A few lamps on the market
spread too much light out into the room and not down into the aquarium where it is needed
for plant growth. This may dazzle people in an otherwise dark room, and it may cause a
cool gleam in the room. Such problems are often caused by hanging the lamps too high, but
may also be caused by a reflector that is inefficient in concentrating the beam. The lamps
must be lowered to a point where all the beams are falling onto the water surface.
In Germany they produce metal halide lamps designed so that only very little light
escapes the open aquarium. Small low-voltage halide spotlights (very popular at the
moment) can provide fantastically beautiful illumination on individual plants. These are
particularly ideal for slant illumination from the front screen inwards, and often give
new glowing colours to both fish and plants because of new reflections. However, these
lamps are usually insufficient as growth lights for aquaria larger than 100 litres.
The leaves of plants growing out of the water may tend to dry out if the relative humidity
of the air is too low, or the lamps are placed too close to the water surface. New shoots
may be sprayed with water a couple of times each day, after which they usually do fine
without any problems. Some plants, however, never adapt to the dry indoor air.
Many plants sold for aquaria - about 50% - are especially well suited for open aquaria.
I will just mention a few of my favourites: Echinodorus grandiflorus, E. cordifolius
and Echinodorus 'Tropica Marble Queen'. The 'Tropica Marble Queen' develops a
heavily marbling above water and is very resistant to drying. All species of Hygrophila
and floating plants will show in all their glory only in the open aquarium. Even
obligatory submerged plants, such as Vallisneria, are better viewed from above.
Some fish may simply jump out of the aquarium. This is partly avoided by using a lot of
floating plants. They provide a great ornamental effect but they may also make some fish
feel more secure. However, some fish are very liable to jump out no matter what you do,
and they should of course be avoided. Tip: glue on a horizontal border of glass (5 cm
wide) all around, and this may prevent even the most jumping of fish from landing on the
Therefore, my answer is "you may safely build yourself an open aquarium!"
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