Swordtail care: swimming room, group dynamics and young fish

The sword is one clue to identity

The sword is the extended lower portion of the tail fin and is characteristic of adult males. Females and young fish lack this extension. The anal fin underneath the rear of the body should also be examined when identifying sex. In males it develops into a narrow reproductive organ called the gonopodium. In females it remains fan-shaped.

Red, spotted and other breeding forms can have more complex ancestry than their colour names suggest. Swordtails and platies can be crossed, as documented in scientific breeding stocks. You should ask how the fish have been identified before buying, particularly if you intend to breed a specific species or line.

Choose swimming length for the adult group

OATA, the UK ornamental aquatics trade association, describes swordtails as usually eight to twelve centimetres long and recommends at least 80 litres. The research database FishBase also records larger fish, including females up to 16 centimetres in total length. Your aquarium plan should therefore allow for larger adults too. The chosen tank should accommodate the group when fully grown.

A long open swimming route should be part of the layout. Plants and other suitable structures should create several sheltered areas along the sides. A fish can then move away without repeatedly swimming through the centre of the group. A tall, narrow tank offers less distance at the same volume. You should seek knowledgeable advice on actual dimensions, the number of fish and any other inhabitants together.

Read more: Setting up a freshwater aquarium: tank, filter and water

Settle on suitable, steady water conditions

OATA lists 22 to 27 °C and pH 7 to 8 for swordtails. The pH value describes how acidic or alkaline water is. General hardness should also be measured. It mainly reflects dissolved calcium and magnesium. You should discuss your readings and the previous conditions of the fish being offered before buying. Hardness figures differ between general care sheets and records of natural habitats.

The filter should be cycled without fish before they arrive. During cycling, bacteria establish and turn ammonia from waste into harmful nitrite and then less toxic nitrate. An ammonia product intended for cycling is used according to its instructions. Repeated checks after the further prescribed additions should show no detectable ammonia or nitrite before the first fish arrive.

Testing and regular partial water changes remain necessary afterwards. Replacement water should be adjusted to a suitable temperature. If tap water contains chlorine or chloramine, an appropriate conditioner should be used. These disinfectants are harmful to fish and the useful filter bacteria.

Read more: Caring for an aquarium: watch the water, filter and fish

Distinguish brief chasing from persistent harassment

Male swordtails can come into conflict with one another. In a mixed-sex group, OATA recommends roughly two females per male to spread his attention. You should still watch whether individual fish are repeatedly chased and rarely get a rest. If a female stays in one corner or cannot reach food, you should review the layout and provide separate accommodation if needed.

For additional species, temperature, hardness, adult size and behaviour should all fit the planned aquarium. Active swordtails may disturb very timid companions by chasing them or competing at feeding time. Before combining species, you should consider where each feeds and can find shelter. Adding more fish is not a dependable way to resolve an existing problem.

Read more: Choosing aquarium fish that can live well together

Make feeding work for the whole group

Swordtails eat both plant and animal material. Suitable complete flakes or granules should provide their daily nutrients. Small portions of appropriate frozen food can supplement them. Pieces should be small enough for the smaller fish to eat too. Food formulated for a different group should not be chosen just because it looks similar.

Offering food in several places can help you assess group behaviour. You can see whether one dominant fish takes everything first or others get a share. Leftovers should be removed and the next portion reduced accordingly. When care is shared, set portions should be agreed so that each person does not feed the group again.

Growing fry need their own space

Swordtails do not lay their eggs in the aquarium. The young develop inside the female and are born able to swim. Some may grow among well-arranged plants without initially being noticed. If you intend to raise offspring, suitable additional accommodation should already be prepared. It needs an established filter and food small enough for their mouths.

As each young fish grows, the aquarium's waste load increases. More feeding also produces more waste. You should follow fish numbers and water readings and arrange suitable new homes in good time. Keeping young fish indefinitely in a small breeding box, a separate container within the aquarium, does not provide enough space for growth.

Look at the fish and their equipment

During daily care, you should watch for changes in swimming, loss of appetite and injuries. A fish staying unusually apart may be responding to group pressure or a health problem. Repeated rubbing, new spots or markedly faster breathing should be assessed by a vet who treats fish.

If several fish suddenly struggle to breathe, they need immediate help. While contacting a suitable vet, you should check the equipment and water readings. A failed filter or heater matters to the assessment, as do recent arrivals. Dates, test results and photographs help you describe what has changed clearly.