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Aquarium problems - overcrowding, moving fish and dimensions

(problems from weight, overcrowding, ... to moving)

Aquarium weight

An aquarium filled with water and substrate is really heavy and this must be taken into account when you plan your aquarium. Even small aquariums can be surprisingly heavy and must be supported by a strong enough structure.

The exact weight of your filled aquarium will depend on several factors, including aquarium type and substrate. Generally speaking, an empty glass aquarium will weight twice as much as an empty acrylic aquarium. (Even a small 20 gallon glass tank can weigh over 25 lbs.) An acrylic aquarium will, despite its lower weight, call for a more supportive stand since it is more flexible and needs to be supported along the full bottom surface. A glass aquarium is heavier but only requires support along the outside edges.

As you begin to fill up your aquarium with water, you will add an extra 8 lbs for each gallon of water. As mentioned above, the weight of the substrate must also be taken into account. A deep sand bed will for instance weigh more than a thin layer of coarse gravel. You can expect your small 20 gallon glass aquarium to weigh well over 200 lbs when filled with substrate and water. Including rocks, stone structures and similar in the set up will increase the weight even more (but it will also decrease the amount of water).

When calculating the total weight of an aquarium, it is easier to use the metric system because 1 cubic centimetre of water has a mass of 1 gram, and 1 litre of water will therefore weigh 1 kg. The water in a 500 L aquarium will therefore weigh 500 kg. If you only know the dimensions of your aquarium, length times width time height in centimetres will give you the mass in grams.

Using gallons and inches is a bit trickier: 1 gallon of water = roughly 8.57 lbs 1 gallon of water = 231 cubic inches

Generally speaking, it is risky to place an aquarium larger than 20 gallons on an ordinary desk or shelf. If your aquarium is larger than 20 gallons, the safest course of action is to use an aquarium stand or aquarium furniture. It is also possible to reinforce a desk or shelf to withstand the weight, but you have to know what you’re doing.

Last but not least, the aquarium stand is not your only concern – you have to make sure that the floor can support the weight of the aquarium as well.

Moving fish

Always be careful when you need to catch a fish, e.g. to move it to a new aquarium. Try to avoid spending a lot of time chasing the fish around in the aquarium because this will be highly stressful – for all the inhabitants.

If you aren’t in a hurry, you can try luring your fish into a trap. For small species you can for instance place some enticing food inside a clean bottle, place the bottle in the aquarium, and wait for the fish to enter. When the fish is inside the bottle, swiftly close the opening with your hand or with a lid. Being caught in this fashion is much less stressful than being chased around with a net. For larger fish, you can use anything “cave-like” and have a larger lid ready.

In aquarium shops, you can usually find both green/black nets and white nets. Green/black nets are harder for the fish to see in the water, while white nets will be perceived as threats by most species. You can therefore use a white net to trick your fish into swimming into a green/black net.

If your fish has elongated spines, netting it can be a bad idea because the fish may become entangled. It is normally safer to use a bucket, strainer or similar to catch such fish.

In some situations, catching the fish by hand is the best option. You should however keep in mind that some species are poisonous and/or electric.

You can make the catching process easier by lowering the water level in the aquarium. This is especially important if you have a large aquarium. (If you go for the trap-method, there is usually no need to lower the water level.)


Overcrowded aquariums are unfortunately a fairly common sight and one of the main reasons so many new aquarists end up loosing all their fish. It might be tempting to add more and more fish to your aquarium, but it is a risky path.

So how much is too much? There are general rules of thumb that you can use, e.g. 1 cm fish per litre water, or 1 small fish per gallon, but they must be used with caution because a lot of factors will influence the ideal stocking density of your specific aquarium. Some species are for instance messy eaters and/or known to excrete plenty of waste and such species typically need much more than 1 litre of water per centimetre fish. Another factor that must be taken into account is your willingness to carry out water changes. If you are prepared to carry out a small water change several times per week you can usually keep a much more crowded aquarium than one small fish per gallon.

There are situations when it is actually a good idea to keep the aquarium fairly crowded, e.g. when housing certain aggressive species that would otherwise become highly territorial and violent. Such fish should however only be kept by well-informed and experienced aquarists that know how to keep the water value up and prevent the other problems associated with crowded aquariums.

Source: Aquatic Community Tropical Fish

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