Fish are very relaxing and beautiful to look at and many species are kept for these reasons. They are suitable pets for people living in flats or other confined areas, but for people with plenty of outdoor space an in-ground fishpond can enhance a garden setting.

Whether kept in an aquarium or pond, a great deal of care is required in looking after fish properly in terms of facilities, time, financial means and level of interest.

Particular attention must be given to water surface area, temperature control and water quality.

Choosing the type of fish you intend to keep

The type of fish you keep will depend on the amount of space you have, and the amount of money you wish to spend on their purchase and the equipment associated with their care.

There are two broad categories of aquarium fish; freshwater and saltwater (marine) – and within each of these categories are coldwater and tropical varieties. Coldwater fish are suitable for coldwater aquariums but tropical fish need heated water.

Outdoor pond fish are normally of the freshwater-coldwater type.

It is important that all indoor fish be kept in a properly set up aquarium and not small bowls. The main reason for this is that the fish bowls have a small water surface area and therefore limit the amount of oxygen entering the water. When selecting fish buy only healthy specimens and be sure to purchase your fish from a reputable dealer (dead fish in the sale tanks indicates poor health). Beginners should start with a freshwater aquarium and a few coldwater fish. Choose hardy specimens such as goldfish (a good variety being a comet and avoid the fancy breeds).

Tropical freshwater fish suitable for beginners include patties, swordtails, guppies and mollies. Be sure that you know how to keep fish properly before buying tropical fish because they are more sensitive to poor water quality and temperature fluctuations.

Saltwater (marine) tanks are more difficult to manage than freshwater tanks. This applies to both cold water and tropical marine tanks. Therefore these should not be attempted by beginners.

Tropical marine fish suitable for aquariums include clownfish, wrasses and tropical seahorses. Seahorses can be difficult to feed properly as they often require specialised food.

Never keep incompatible fish together as some may eat others. Also, be careful with aggressive fish such as Angel fish. They can be very aggressive towards each other, particularly if the space in the tank is limited. For advice talk to a reputable dealer and also contact your local Aquarium Society. Members are usually very helpful with advice, equipment purchases and sourcing of suitable fish.

Care for your fish

Setting up an aquarium

A fish tank should be an appropriate size for the number of fish held. If a non-aerated aquarium or outdoor pond is used, allow three square centimetres of water surface area for every three square centimetres of fish (including the tail).

A tank should be rectangular; fish bowls have a small water surface area and therefore limit the amount of oxygen entering the water. In addition, for each centimetre of fish you will need about five litres of water regardless of whether the tank is aerated or not. This figure is only a guide and will depend on the type of fish you keep – seek professional advice if uncertain. The volume of your aquarium in litres can be calculated by using the following simple formula. All measurements are in centimetres.

Volume = (length x depth x width)/1000.

It is now normal for glass tanks to be glued together with silicone sealant rather than having a frame as in the past. If a frame is used, marine aquariums must be framed with aluminium or stainless steel to prevent damage to the frame by the saltwater. Either way, the glass from which the tank is made must be thick enough to withstand the weight of the water.

Tap water is potentially dangerous to fish due to the chlorine that it may contain. Therefore any new water added to the aquarium should be treated with a dechlorinator or conditioning salts (available from aquarium shops, see below). Seawater is best made up from bottled water or tap water with the addition of special sea-water mix available from aquarium shops.

Step by step instructions

Place the empty aquarium and stand on a flat solid surface in a well-lit position but not in direct sunlight. Direct sunlight will heat the tank water too much during the day and it will cool too much at night and as a result your fish will suffer stress diseases. Direct sunlight will also cause too much algae to grow and the tank will be difficult to keep clean.

Remember that one litre of water weighs one kilogram and there is the additional weight of the tank, stand and other equipment. The floor surface and stand you have chosen must be able to support this total weight.

First, half fill the aquarium with water (clean tap water is suitable). Use a new clean plastic bucket that has been rinsed and washed with tap water before you use it. Keep this bucket solely for your aquarium use. Do not use the laundry bucket as this may have traces of chemicals that may harm the fish. If a bottom filter is to be used, install it at this stage.

Then place sand on the bottom with pebbles and stones for decoration and to provide hideaways for the fish. Anchor in some water plants, which will help, oxygenate the water.

Complete the filling of the tank to within five centimetres of the top. Fish may jump out of the tank, so cover the aquarium with a glass top that is raised sufficiently to let air in.

It is important to add dechlorinator or conditioning salts at the beginning of this process.

A water filter operated by a small electric submersible motor will help to keep the tank clean, but if your fish are being over fed (thus excreting too much ammonia), algae may still overgrow in the tank.

The same motor can operate an aerator that pumps tiny bubbles of air into the water. These aid in circulating the water (which helps with oxygenation, since most oxygen exchange takes place at the surface). Bubbles also show that the filter is working properly. Many filters use a combination of charcoal filters and fine sponge filters to remove solids. These will need to be regularly cleaned (using old tank water not tap water) and replaced as needed.

For tropical fish a heater and thermometer are also needed to keep the water at the right temperature
(22° C – 24° C for tropical, freshwater and marine fish and 15° C – 20° C for coldwater fish). The effects of central heating and other forms of room heaters should be considered. Water temperature in tanks should be checked daily and maintained within a range of 5° C of the optimum temperatures.

Let the newly filled aquarium stand for about 3-4 days before buying your fish. It is important to add dechlorinator or conditioning salts at the beginning of this time.

Fish are usually sold in a plastic bag and this should be floated on the surface of the tank water for about 30 minutes to allow the water and fish in the bag to reach the same temperature as the water in the tank. This way, fish will not get a shock by being moved to water at a different temperature. A sudden temperature change of even a few degrees can kill the fish. After this time, add tank water very slowly into the bag to allow the fish to acclimatise to the tank water (and thus prevent osmotic shock). Leave the fish for another 30 minutes in the bag and then carefully pour the fish into the tank.

New aquariums take at least six weeks to ‘settle down’ after the fish have been added. Fish excrete ammonia into the water, which is potentially toxic. This ammonia is converted to nitrite (which is also toxic to fish). The nitrite is then converted to more harmless nitrates. ‘Nitrifying bacteria’ help both these reactions occur and it takes up to six weeks for the levels of bacteria to stabilise. The plants use the nitrates in the water. During the settling in time tanks should be tested each day for ammonia and nitrite (using kits which can be purchased cheaply).

This settling in period can be handled in a few ways as detailed below.

Options 1 and 2 – after you have introduced the fish:

  1. Regular water changes. Change about a quarter of the water every week whether it is dirty or not. Be very careful to ensure that the appropriate replacement water is at the same temperature as the tank (to within two degrees). Always add dechlorinator or conditioning salts to the water whenever you do a water change. If the tank is particularly dirty replace about 1/3 of the water at once, clean the filter and then remove and replace a quarter of the water each day until it is clear again. Carefully monitor the ammonia and nitrite levels.
  2. Commercially available bottles of ‘nitrifying bacteria,’ as these will convert the toxic ammonia and nitrites to the less toxic nitrates.Option 3 – before you have introduced the fish
  3. Fishless cycling – involves the addition of ammonium compounds to a tank containing no fish to enable the ‘nitrifying bacteria’ to develop before any fish are added.

Establishing an outdoor pond

Fish ponds are best if in-ground and made of cast concrete, although some prefabricated cement constructions and some butyl rubber liners are suitable. Where plastics are used, ensure that the supplier warrants them suitable for use with live fish, as many plastics are toxic. The minimum depth of available water must be 30 cm, but it is wise to have even deeper sections so that fish can reach cooler water areas. As with aquarium fish, the number placed in a pond will depend on the surface area of the water and whether or not the water is filtered and circulated.

Adequate aquatic plants must be placed in the pond to help remove nitrates and phosphates (which cause algal growth) and to provide shelter for the fish and a place for eggs to be laid. Algae and plants generate oxygen during the day, but absorb oxygen at night. On still warm summer nights the plants and algae can easily remove all of the oxygen and kill the fish. To avoid this, ensure that pond pumps run at night when there is no wind to ruffle the water. Rocks must also be strategically placed to provide good shelter, enrichment and for decorative purposes.

Ponds are best sited out of direct sunlight in order to control algae growth. Watch for predator birds if setting a pond near large trees. A water filter system is essential to remove algae and other impurities and the system should be designed so that one-quarter of the water is replaced each week. Newly poured concrete ponds, and new butyl rubber liners should be filled with water and allowed to stand for four weeks. After draining and cleaning it is safe to refill and stock with fish.


Never overfeed the fish. If your fish are kept at a constant temperature in the aquarium you should feed them every day. Allow just enough food for the fish to feed for about two to three minutes. Supply a variety of dried food (flakes), frozen food (brine, shrimp, daphnia) fresh food (earthworms, tubiflex) and green food (algae and water moss) all of which is available from aquarium shops. Do not overfeed fish as the uneaten decomposed food will make the water smelly or clouded and the fish will die. Carefully siphon off any uneaten food from the tank floor.

Pond fish require feeding every two days in summer and once weekly in winter. Buy suitable pellets from a pet supplier. Trout pellets are generally too rich in protein for fish such as carp and goldfish, and can cause pollution problems in the pond.


The principal causes of death of fish in an aquarium or pond are overstocking and polluted water. As fish must live permanently in the area where they eat and excrete, the tank must be cleaned regularly to remove this material, which will foul the water. Learn to recognise normal fish behaviour, and know what to do if the fish begin to behave in an unusual way. For example, fish gasping on the surface is a sure indication of a fouled tank or pond with little oxygen left. Other signs of infections and diseases are change of colour, swollen skin or eyes, a swollen belly, a rotting tail or white spots over the body of the fish. Check with your veterinarian for advice.

Remedies for some common problems such as white spot, fin rot and fungus are sold from aquarium shops. But always check with your veterinarian. Prevention is obviously better than cure, but where a fish becomes sick it is best to remove it to a small treatment tank so that the other fish do not become infected. It is a good idea to have a small spare tank that can be used as a treatment tank if required. Seek advice about treatment from the local Aquarium Society or veterinarian.

Remember that tobacco, dog and cat flea treatments and garden and household sprays such as insecticide can pollute the aquarium or pond and kill the fish. Read the labels carefully and seek advice. Always cover the tank or pond before spraying and turn off any aerators.

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