by Andy Horton

"What have you got in there?" I asked peering into the bucket that contained a mixture of small fish, crabs and prawns. It was a large 3 gallon bucket with a wire carrying handle, that had been placed in a cool pool in the shade. The water in the bucket had been collected from the sea below low tide limit rather than the pools which may have already deteriorated in quality.

This is most elementary type of temporary aquarium. However, even for short periods of less than an hour, the fish and crabs can succumb in conditions that are not suitable for even the hardy marine creatures that can be found between the tides.

Life in a Bucket

Why is this? Afterall it is the same water that the animals were collected from. The answer lies in the changes that occur in the bucket. The two most important factors are:

Oxygen Depletion

Fish and marine invertebrates obtain oxygen from the water in which they live. The oxygen is renewed from the atmosphere at the water's surface. Oxygen becomes depleted in the bucket because the animals consume it quicker than it can be renewed from the atmosphere, and the fish will gasp at the surface and eventually suffocate unless they are returned to the sea or remedial action is taken. This is further compounded by the invisible plankton in the sea water that dies and puts further demands on the available oxygen.

Temperature Amplitude

Marine life can only live in a narrow temperature range that varies from creature to creature. However, even the highest temperature that the seas around Britain get rarely exceeds 19 ° C. About 50% of the rock pool fish and crabs are unable to endure temperatures more than a few degrees above this for long periods.

In a bucket on the shore it only takes a few minutes for the water temperature to rise to the same as the air temperature which last summer was much too high (for aquaria) at 28 ° C, but even temperatures of 25 ° C may be too high for some of the northern species.

Life Support Systems

It is unethical to take home creatures from the shore and the sea unless you have a properly set-up aquarium to put them in. Without any life support systems the tank is no more than a giant bucket. It can only be called an aquarium when the aeration, filtration and temperature control systems are installed. A cooler may also needed if you wish to keep a wide range of British species.

This is not the only thing to think about. Fish and invertebrates have to be fed the correct and sufficient amounts of food and attention should be paid to the size they will eventually attain. Compatibility needs to be considered. Fish will not only eat each other, but some of the more aggressive animals may be quicker on the uptake and deprive shyer fish of their share. Finally, the sea water in the aquarium needs to be replenished at monthly intervals.

Sea Water

Tap water mixed with table salt will not do. Sea water contains a large number of additional mineral salts of which many are essential to support life. In addition to sodium chloride these include magnesium, sulphur, potassium, calcium, potassium, bicarbonate and a lot of minor and trace elements.

The aquarist has the choice of buying special proprietary 'marine salts' from aquarium shops, and mixing the contents with tap water according to the instructions on the packet, or collecting the real stuff from the sea.

Artificial Sea Water

These salts (make sure that you explain to the retailer that the salts are required for marine aquaria) prove to be suitable for keeping British marine life in captivity. The salinity should be 3.4% which equals a specific gravity of 1.025 at 15 ° C and 1.024 at 20 ° C. An error of more than .002 should not be permitted.

Real Sea Water

Apart from the heavy weight, the real stuff actually presents more problems and is generally only used when an aquarium is started or for replacements in invertebrate only tanks.

The following points are worth bearing in mind:

1) Collection should be from an unpolluted source.

2) Estuarine water needs to be checked for salinity.

3) Real sea water contains vast amounts of plankton. In times of 'bloom' when the numbers explode, the introduction can cause a serious oxygen shortfall in the aquarium.

4) Containers should be clean.

5) Sea water should be used immediately, or stored in the dark for one month or more. Sea water kept for over one day has proved harmful by experience. This could be for a number of reasons including bacterial explosions.

6) Pathogens, including free swimming parasites can be introduced.

7) Suspended particles of silt can be present in appreciable numbers, even in water that does not look murky.


Fish and invertebrates respire and like humans they inhale oxygen and exhale carbon dioxide. Unlike humans they extract the dissolved oxygen present in the water. The oxygen would be all used up very quickly if in the sea and the aquarium it is not replaced by oxygen from the air. In still waters like a bucket, this process is not very quick.

Aeration in aquaria is designed to increase this gas transfer. The best way to do this is to increase the surfaces by which oxygen can enter the water, which is done by increasing the turbulence at the water surface. This can be achieved in lightly stocked aquaria by the use of the filtration equipment which circulates the water, and can agitate the water surface by a variety of methods including spray bars. It is good practice to use an additional powerhead (water pump) with an air intake tube for the purpose of disrupting the water surface and providing supplementary currents and circulation in the aquarium.

Biological Filtration

Wastes are expelled directly into the water in the forms of organic waste, urea, ammonia etc. Ammonia is excreted by fish and invertebrates and results from the break down of organic matter. Ammonia is toxic to fish in amounts too small to be accurately tested by the home aquarist. In conventional marine aquaria the answer to this problem in bacteriological control whereby a suitable medium is provided to provide a home for helpful aerobic bacteria called nitrifiers which convert the toxic ammonia into less toxic nitrites and to nitrates.

The usual medium is gravel or coral sand which is used in the undergravel system of filtration where a powerhead is used to circulate the aquarium water by drawing it through the gravel. The nitrifying bacteria live amongst the gravel. In the external system, the gravel, or alternative medium, is housed in a canister. Biological filtration should be operated continually. It is not satisfactory to turn the filter off at night.

Undergravel System

Two substrates can be used separated by a mesh called a 'Gravel Tidy'. The coarser 'calcium plus', crushed maerl etc. is put underneath with the top layer of coral sand, or even beach gravel. The size of the gravel particles should be between 2 to 4 mm. This is the size of coffee granules. The depth should be 40 mm or more.

Link to large diagram of an aquarium (takes ages to view)

This is the simplest type of filtration. The disadvantages of undergravel filter plate system is the uneven flow that can occur through the coral sand and the difficulty of cleaning. The filter bed is also beneficial as it acts as a sieve extracting particles from the water and keeping it clear. However, this dirt remains in the coral sand and over a period of six months or more tends to obstruct the flow of water. The remedy is to clean the gravel as part of the maintenance programme using various siphon-like devices. Powerheads (water-pumps) are to be preferred to the air-pumps.

External Filter

The filter medium is kept in a separate canister and the water is pumped out of the tank and through the filter medium and back into the tank again. The pump is usually contained in the canister. There is very little equipment in the aquarium, the pump can be disconnected for easy cleaning, and the canister can hold a variety of special filtration mediums. Internal canister filtration is also available.

Adsorptive Filtration

Organic waste can be removed directly from the aquarium by adsorptive materials like 'activated carbon' and by the use of the Protein Skimmer. They are supplementary methods and should be used in addition to biological filtration. The main advantage is they delay the build up of the biomass in the aquarium and should only be installed after the aquarium has been installed for 6 months, or alternatively, they can be used after feeding only.

New Aquaria

In the week of setting up an aquaria, the amounts of ammonia, nitrites, and dissolved oxygen can fluctuate wildly. and only the very hardiest of rock pool animals like Shore Crabs and prawns should be kept. After about three weeks the aquarium will settle down and more animals can be introduced.

Established Aquarium

After between 3 and 6 months the filter bed settles down and the nitrifying bacteria are operating at their optimum. The tank can then be called 'established'. Experienced aquarists realise that an aquarium is a constantly changing environment and has to be continually monitored. The following events occur:

1) Water evaporates and needs to be replaced by tap water. A hydrometer is required to check the specific gravity.

2) The pH decreases as a result of actions of the aerobic nitrifying bacteria. The ideal pH is 8.2 which can be measured using test kits.

3) The aquarium water changes with a build up of nitrates, a yellowish colouring caused by 'dissolved organic carbon' (DOC), and the balance of salts change.

Events 2 and 3 can be best be remedied by 25% monthly water changes of sea water.

4) The biomass of the aquarium increases with the growth of animals and multiplying of microscopic organisms and bacteria. If this is not watched and the gravel cleaned removing some of the microorganisms, there is a danger of a dissolved oxygen shortfall.


The seas around the British Isles are often murky and light levels are less than tropical seas. For all the fish and most of the invertebrates, except for a few species of sea anemones, a single 40 watt fluorescent aquarium lamp may only be the minimum needed. Aquarists often use a combination of a blue fluorescent like a Triton lamp in addition to a standard unit.

Temperature Control

Native marine aquaria differs from the tropical hobby by the need to make sure that the temperature in the aquaria does not get too hot (over 22 ° C) during the summer. If the aquarist is not lucky enough to have a cellar, or a cool spare room or the luxury of 'air conditioning', he will have to instal a 'cooler', or return the intolerant (about 50%) of the animals to the sea.

Single Outlet Beer Cooler
Special 'titanium' coolers are available at a high cost. However, most aquarists use 'beer coolers'. A powerhead is used to pump the water through the cooler. For a 4 foot aquaria holding about 15 gallons a single outlet 'cooler' is sufficient. For larger aquaria a double outlet cooler is necessary and will handle up to 50 gallons with a temperature drop of 10 ° C. thermostats are rarely accurate enough. For precision, and electronic aquarium thermostat designed for heating equipment can be adapted by the manufacturer, before purchase, for use to control the maximum temperature.

Further Notes on Coolers Link

Feeding & Disease

Rock pool fish and crabs will eschew flake food and in any case it is unlikely to be suitable as a sole diet. Therefore, the aquarist needs to feed the various fish and invertebrates what they would eat in the wild. Fortunately, many of the rock pool inhabitants eat a variety of foods. Mussels, cockles, prawns, shrimps, limpets, ragworm, sandhoppers, small crustaceans, zooplankton, mysids and winkles are regularly collected for food. Mussels are boiled in their shells, and mysids are used as live food. Pathogens can be introduced with live food and in this respect winkles are the biggest culprits.

Wild caught fish inevitably harbour parasites. Most of these are invisible and even visible parasites are rarely fatal. Less than 1% of fish caught will die as a direct result of infection. Small fish will sometimes scratch their bodies on rocks. Recent study has proved that in many cases this is not a sign of parasites, and this behaviour has been observed commonly in the sea.


As far as the fish and sensitive invertebrates are concerned the most hazardous part of the existence in captivity is likely to be the journey to and from the shore. The problems encountered in the bucket are multiplied the longer the journey. The following tips may ensure success:

1) Temperature can be prevented from rising too quickly by the use of polystyrene boxes.

2) The problem of plankton blooms and resultant oxygen shortfall can be overcome by transferring the fish to a container of freshly mixed artificial sea water.

3) Oxygen levels can be increased by the use of a battery operated air pump and airstone, or a 12 volt 'continuously rated' water pump.

4) A large container with the greatest surface area of water as possible should be used.

Captures should not be introduced into aquaria with more than a 2 ° C different in temperature from which they are originated. Fish should not be acclimatised by placing in floating plastic bags - this is more likely to do more harm than good. Quarantining of fish is not normally necessary, although aquarists with several tanks may put new fish into supplementary aquaria rather than their main display tank.

For more information access Wet Thumb

British Marine Life Study Society


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