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Issue: March 2007

Author: Phillip J. Brown

The very image of an aquarium today conjures up a rectangular box with a lid. Though this might suit our modern ideas and our box-shaped rooms, an aquarium can be round or octagonal or any other shape that modern glass, and in particular acrylic, can create.

The very image of an aquarium today conjures up a rectangular box with a lid. Though this might suit our modern ideas and our box-shaped rooms, an aquarium can be round or octagonal or any other shape that modern glass, and in particular acrylic, can create.

It could be said that the modern closed-top aquariums prevent viewing the fish and plants from above, a view that pond keepers enjoy all the time. Watching the fish and the rippling water from above can be very satisfying. Antique aquariums were often designed to be viewed primarily or even exclusively from the top.


The open aquarium uses suspended (pendant) lights to take the lights out of the way. These are used by plant enthusiasts for the more powerful halide bulbs that run too hot to be kept close to the water surface. They are suspended from the ceiling by hooks and need to be hung at the correct height so that the light falls directly into the aquarium and not spill into the room itself.

A newer option is to use a fixture which attaches to either side of the aquarium. These are not so convenient if you have plants growing out of the tank but look superb on a marine aquarium. Some have moon lamps for nighttime viewing, which can add greatly to the enjoyment of the aquarium in the evenings—and in some marine species can stimulate spawning. Most have a fan for cooling. Splash lenses protect the lamps from corrosion.

A cover can still be placed across the top of the aquarium, but the true open aquarium leaves the top exposed. This way the floating leaves of the plants, and in some species their emersed leaves, can be seen, and the fish can be viewed from above. Some aquarists even grow plants up and out of the aquarium. Many terrestrial or bog plants will thrive with their roots in an aquarium.

What About Evaporation?

In a small room an open-top tropical aquarium can adversely affect the overall humidity of the room, leading in some cases to dampness and windows running with condensation. However this is not a problem in a larger room or where ventilation or humidity convectors are used.

One of the biggest factors is the difference in temperature between the aquarium water and the temperature of the room itself. If the room is unheated, water vapor will condense out. In an averagely heated room this will not be noticed, and the slight increase in humidity often feels beneficial in a dry, centrally heated room.

Remember topping off the aquarium to replace evaporation is not the same as a water change. Any toxic constituents do not evaporate, and adding straight tap water will only gradually add to these, over time leading to a dangerous level that can severely affect your fish. A regular schedule of water changes with properly conditioned water and adequate filtration and monitoring will prevent any problems before they build up.

If the tank needs topping off in between, use water that has been through a reverse osmosis unit, distilled water, deionized water, or rain water (if from a pure source).

Jumping Fish

Some fish like to jump, and it is obviously upsetting to find a fish dead on the carpet. Some fish are more likely to jump than others, and in an open-top aquarium it is best to avoid well-known jumpers. Remember that while surface dwelling fish like hatchetfish and swordtails are quite prone to leaping from the tank, benthic (bottom dwelling) fishes like loaches and eels are also generally poor candidates for an open aquarium, as their habit of sliding up and over obstacles in their path can take them right up the side of the tank and out over the rim. Snails may also crawl out, and in the marine aquarium, crabs are master escape artists.

Some fish may also jump when first added to an aquarium or in the first few weeks as they adjust. A temporary cover can be used for a few weeks if this likely to happen. Also, a mass of floating plants (this can be reduced later if required) helps to shade the fish and can reduce jumping. A well-planted aquarium also helps by providing shelter and reducing the effects of sudden movements inside and outside the tank. Fish may also jump at feeding time, when they can get very excited. On the other hand, an open aquarium makes feeding a fascinating experience, seeing the fish schooling and feeding from above.

Gluing a strip of glass or plastic around the edge of the tank about 2 inches (5 cm) wide will prevent most jumping problems, as will a full rectangular cover with a large rectangle or circle cut in it.

An Ounce of Prevention

An open top helps exchange of gases between the water surface and the atmosphere. This also means, if running an open top aquarium, you, your family, and any visitors should avoid such things as smoking and spraying chemicals into the air. And of course you should be more careful not to drop things into the aquarium. But really, these are all sensible precautions for any aquarium. The open-top aquarium is also not recommended around inquisitive cats!

The disadvantages and extra precautions aside, once you have seen your fish living from the top, it adds a whole new dimension to your fishkeeping.


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