Aphids are the most common pest and can be hosed off the plant so the fish can eat them.  You can also fully submerge the whole plant under water for a day, as the aphids can’t swim.  Lady bugs are a natural predator of the aphids.  If you notice them around your pond, that is a good sign to let nature do its thing.  Sometimes the aphid infestation may be too great.  In these instances you may need to use a pesticide.  We recommend that you take the plant/s away from your pond (preferably somewhere shady) for treatment.  After treatment, wait a day or so, rinse with water and return them to the pond.  More delicate plants may need to be placed in plastic bags to prevent drying out.  We are not aware of any pesticide manufacturer that currently endorses their product as being safe to use in an aquatic environment.  However, as commercial growers of aquatic plants, Oz Watergardens has been using Confidor and Rogor (both systemic insecticides) for aquatic pest control.  In all our years of use, we have not found any problems with our goldfish.
Warning:- Don’t spray with any pyrethrin-type insecticides on your water plants or in the garden too close to your pond as pyrethrin WILL kill fish.

Mealy bugs can also be dealt with in the same aquatic pest control measures as aphids. 

Spider mites have become resistant to many water pest control treatments.  If you have a problem, submerge the affected plant for a couple of days.

Caddis Fly larvae, which attack the underwater leaf stems of waterlilies and other aquatic plants, are best controlled biologically by introducing some fish into the pond.

Mosquitos and other insect life can easily be controlled by fish.

Caterpillars, grubs and other forms of larvae are best dealt with as fish food or just squash them.

Snails  – Most are pests, although not really a major problem unless their populations really blow out.  The Ramshorn snail is an exception, as it is a good algae eater.  The American ribbed fluke snail is a commonly found fresh water snail, with a pointed, conical-shaped shell.  This snail is a potential carrier of liver fluke and its spread should be minimised to reduce the risk to live stock, such as cattle.  It is therefore illegal to bring aquatic plants from the East side of Australia to Western Australia due to quarantine restrictions.  Certified aquatic plants can only be sent across if they have been treated, inspected and quarantined.

Pond snail numbers can be reduced by floating lettuce leaves in the water, then collecting them when the snails attach.  Blue stone copper sulphate (250g Copper/kg) can be added to the pond water at a rate of 4 grams per 1000 litres of pond water.   That is equivalent to a copper level of 2ppm (parts per million).  At this concentration the copper is toxic to the snails (and also algae), but not harmful to fish or other pond plants.

Leeches are not as scary as they seem.  Most are only found at the bottom of the pond.  There are no known aquatic pest control treatments that we are aware of.  What we have noticed, is that they seem to appear for a year or two and then disappear again naturally.  We have heard of suggestions like placing a piece of meat in the pond water and then collecting the meat when the leeches attach.  It may work, but we’ve never found it all that successful.

Crown rot, though not common, is a devastating disease that affects some waterlilies.  Leaves turn yellow and no new growth appears during the growing season.  Eventually the crown of the plant rots.  There is no cure and plants infected should be removed before it spreads to other plants.

Frogs – You might ask why we have included frogs here, however, some people complain that they become too noisy at night.  Our only recommendation would be to avoid placing your pond beneath your bedroom window, if you think that they might be a problem.  Remember, if you build it, they will come. 

Tadpoles are herbivores and most only eat algae, however, some have been observed eating softer leaved aquatic plants such as the floating leaves of waterlilies.   We have not seen this, with the 7 species of tadpoles at our nursery in Victoria, but have seen them eating away at the outer edges of waterlily leaves in Sydney.  We can only assume that this occurs with a particular species.  If this is a problem and the waterlilies don’t survive we can only suggest that you select emergent foliaged aquatic plants.

Prevention is the best solution to water pest control.

Healthy vigorous plants are always more resilient to attack from pests.  Maintaining an ecologically balanced healthy pond is the key to reducing water pest control measures.

Dragonflies and damselflies are actually beneficial to the pond.  They actually prey on other insects.  Therefore, the more dragonflies the less insect pests.  Large fish may be a problem for the dragonflies, as they love to eat the dragonfly larvae and nymphs.  Many fishermen also love to use them as bait for trout.