The open aquarium

by Claus Christensen

The open aquarium - without the top glass - with the lamps hanging from above provides a new dimension of the traditional aquarium and it opens up for plants, which produce an emergent inflorescence. Does it introduce problems with humidity in our homes? Will the leaves of emergent plants dry out? What will happen to the fishes?

The open aquarium is my favourite aquarium! It provides new dimensions for the aquarium and the living room. We can observe the leaves of the floating plants, the flowering aquarium plants such as Nymphaea (water lilies) and a lot of aquarium plants are amphibious and grows out of the water where they provide great ornamental effects. In addition, open the aquarium offers more possibilities for a more exciting illumination. But the beautiful aquarium may affect the indoor climate much more than the traditional aquarium.
Relative air humidity A frightening example comes from an aquarist holding an open aquarium of 800 litres in a room of 16 m2. He had Discus in the aquarium and the water temperature was 28oC (82oF). Problems cropped up because of the large aquarium and the huge evaporation caused by the relative great difference in temperature between the room and the aquarium. The wallpaper got mouldy and the window glass was constantly misty. Usually open aquaria do not cause problems with damp if the following is 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 5oC (9oF) to keep evaporation at a minimum. We have seen problems in un-heated rooms during the winter because the evaporation increases tremendously with the difference in temperature. Also, 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 decrease the evaporation because they create a boundary layer of relatively stagnant air! If these precautions are taken, the relative humidity will increase by about 5%.

I have asked scientists working with health and air humidity and recent research have shown that people cannot feel variations within 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 gasses released from materials in the room, e.g. building materials, carpets, paint et cetera. Thus, people usually do not feel the increase in relative humidity that an open aquarium causes. For comparison, one person emits 2.5 to 3 litres of water per day (breathing, sweat, showering, cooking), and if the open aquarium evaporates some 15 to 20 litres per week (an aquarium of 800 to 1000 litres) corresponds to an extra person living in the home. Some allergists are very sensitive to microorganisms and the substances they may emit. But recent research has shown that the growth of the microorganisms is independent of the actual relative humidity. The 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 - has 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 referred to diverted and indirect 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, psychologically in nature.

Illumination
Different types of lamps can be used for the open aquarium. There are some disadvantages from a few lamps on the market that are spreading much of the light out in the room and not down in 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 it may also be caused by a reflector, which too 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, which is so thoroughly made that only very little light escapes the open aquarium. Small low-voltage halide spotlights (very popular at the moment) can provide a fantastic beautiful effect illumination on individual plants. In particular, they are ideal for a slant illumination from the front screen and inwards - this often gives new glowing colours to both fish and plants because of new reflections. However, these lamps are usually too insufficient as growth light for aquaria larger than 100 litres.

The amphibious plants
The leaves of plants growing out of the water may tend to dry if the relative humidity of the air is too low or the lamps are placed too closed 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, does never adapt to the dry indoor air. A great deal of all 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.
The fishes
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 broad) all way round as this may prevent even the most jumping fish from landing on the floor. Therefore, my answer will be "you may safely build yourself an open aquarium!"