Fish are an important part of a garden pond’s ecosystem. They eat aquatic pests and mosquito larvae which breed on the surface of even the smallest amount of still water and of course provide movement and colour to the pond. As far as pets go, they are quite low maintenance and cost very little to keep.
Although there are many options, goldfish are by far the easiest or all pond fish to keep. In particular, the common goldfish and comet varieties are terrific as are the multi-coloured Shubunkin. Goldfish are quite hardy in all but extreme heat and cold climates and will easily adapt to their situation. The common goldfish kept in a fish bowl is the same as that of the garden pond but will grow bigger with fewer restrictions, anywhere up to 30 centimetres.
Even the common goldfish has variety in their markings from white through pink and orange to red. Their markings will change as they age and sometimes because of the temperature of the water that they are in. Comets and Fantails are varieties of goldfish which have different shaped tails and bodies but are just as hardy as their more common cousins.
Shubunkins have a similar sleek shape to the common goldfish but have a rich array of colours including mottled whites, orange, yellow and blue combined with black spots. Not all varieties of goldfish are suitable for all ponds as some more developed kinds are more vulnerable to damage and attack by predators and need more controlled conditions.
Goldfish are well suited to garden ponds that include plants as, though they will have a bit of a nibble at the leaves, they generally don’t do a lot of damage and enjoy hiding in amongst the foliage. If their nibbling becomes a problem, choose different plant varieties or use fine wire to make a cage to put around the tastiest plants below the water surface.
A filter is important to keep the water clear and to remove waste and where a filter is added, plants are not essential. However, oxygenating plants should be included as they raise the oxygen levels of the water.
A goldfish pond should be at least 60 centimetres deep and even deeper in areas prone to freezing. During winter, goldfish will become slow, stop eating and stay towards the bottom of the pond. This is normal and they will become active again in spring.
A goldfish pond should always include places for the fish to hide from predatory birds. This can be old pots placed on their side or rocks with holes in them but the inclusion of mesh across the surface will help keep them a bit safer. Of course, this is also an important addition to help keep children in the garden safe around the water.
Goldfish are opportunistic feeders meaning that they will eat whenever the food is available whether they are hungry or not. This can prove fatal as if they are fed too much as they will continue to eat until they become jammed with food. Goldfish should only be fed as much food as they can consume in 3 to 4 minutes and no more than twice a day. They require a food that is lower in protein but higher in carbohydrates so choosing a specific goldfish food is best.
Most goldfish will breed quite easily in a pond and often in spring once the weather begins to warm. They produce eggs that attach themselves to aquatic plants and hatch within two to three days releasing tiny fry only a few millimeters long. Within a week they will begin to look more like goldfish but it can take up to a year for them to become their mature goldfish colour. In a pond, goldfish can live from 10 to 20 years and occasionally longer.
Native fish are not easily available but could be considered by those that are keen to allow frogs to breed in the pond since ornamental fish tend to eat their eggs. It is illegal to catch your own fish from waterways and in fact, this is fraught with danger since some small fish that are mistaken for being native are in fact the aggressive Gambesi.
Western Pigmy Perch occurs naturally in the Perth metropolitan area and the south west of WA. These fish grow between 5 and 15 centimetres long and are usually a greenish brown colour with gold patches. They live for about five years in captivity and are terrific at controlling mosquitoes and midges without interfering with the frogs that may also be breeding in the pond. They don’t generally need additional food but can be offered “goldfish” food if necessary.
Western Minnow have similar needs to Pigmy Perch but are much more animated and more visible because of their silver colouring. They do have a habit of launching themselves out of a pond so high sides are essential.
After goldfish, Nishikigoi or Koi are the next most popular of pond fish. They are relatives of the goldfish but are stockier in shape and can grow up to 80 centimetres. Their remarkable colours, temperament and hardiness make them terrific fish for the hobbyist. However, they are boisterous and can wreak havoc on the plants in a decorative pond and this is why it is far better to keep them in a pond all of their own.
Koi will quickly learn to recognise the voice and footsteps of the person that regularly feeds them and will even take food from the hand. They are calming to watch, live for up to 100 years and are considered companion pets by those that keep them.
Koi are great pond fish but are banned in all Australian states except WA and NSW because of the potential risk of them being released into the waterway and threatening the native fish in the region.
Koi can be kept in almost any size pond but smaller areas will stunt their growth and health. In general, to keep eight healthy fish the pond should have the right filtration and be no less than 3 metres long, 1 to 2 metres wide and at least a metre deep. To produce fish larger than 30centimetres, a much larger pond is required.
A koi pond requires a filter and pump that runs all year round but these are quite inexpensive to operate and will help the fish to reach their optimum size. In general, the more biomass of fish in the pond, the larger and more efficient the filter needs to be. Commercial koi food of a very high quality is available which makes feeding the fish easy and improves their health, colour and growth. They also love cooked vegetables as well as salad vegetables, wholemeal bread and cooked porridge but be careful not offer too much as this may cause the water to become dirty. Avoid animal fats and be aware that their appetite tends to increase in the heat of summer.
Koi will spawn naturally just before summer but often the parents will eat their own eggs. To breed koi, the eggs have to be collected quickly and raised in a second smaller pond.
To find out more about building the perfect pond or planting pond plants check out the other fact sheets on the Better Pets and Gardens website.
Freshwater snails are often added to ponds to help clean the algae from the edges but in a garden pond they can become more of a pest than a benefit. Some species will breed quickly and if numbers get out of hand they will damage plant foliage. In a pond where there is adequate stock of fish, severe infestations are less likely as the fish enjoy eating the snail eggs and even the immature snails too. It really is a case of balancing the numbers and choosing local species that are less likely to cause environmental problems.
DEALING WITH BLANKET WEED
There are a variety of different species of filamentous algae such as blanket weed that affect ponds, clogging the filters and becoming tangled with plants. A certain amount of algae is expected in any pond but will become excessive due to a build-up of nutrients or other factors upsetting the balance such as an increase in water temperature and level of sunlight during spring and before the plants have been able to recover from winter. If this is the case, the pond may balance itself out again once the plants have re-established but more often than not, it will need a helping hand.
Start by physically removing the blanket weed, winding it onto a branch but leaving the algae on the side of the pond for any living creatures that remain. Then take a water sample into any Better Pets and Gardens store and the staff will identify any obvious deficiencies in the water and offer advice on how to correct it. They can also help with commercial treatments that will remove any excessive sludge from the pond and will correct the nutrient imbalance responsible for the uncontrolled growth of algae.