Despite the fact that snails are beautiful animals, there are situations in which they are unwanted.
This could be for example when there is an excess of snails in an aquarium. Also it's a fact that not everyone is pleased with snails and even consider snails to be harmful and some species do eat your aquatic vegetation. However, it's important to realise that snails can play an important role in maintaining a healthy eco-system in an aquarium: they remove dead fish, they eat decaying vegetation, clean up food excesses and snails as the trumpet snails are beneficial for the bottom quality.
The snail population should stay relatively constant and an excessive growth in the snail population indicates possible problems in the balance of your aquarium (primary a food excess). 
Another occasion in which snails are unwelcome is when they treat human food production. In the case of apple snails this applies to area's where they are artificially introduced (e.g. Pomacea canaliculata snails in Asian and Hawaii) where the apple snails can cause serious damage to rice fields and taro patches.
Furthermore, you might want to kill a snail for scientific/educational reasons (dissection of the snail to learn more about invertebrate anatomy) or to reduce the spread of snail transmitted parasites that can affect humans and cattle.

Aquaria and ponds

Prevention

Controlling the population

Manual removal
Hand picking won't eliminate all snails, but it's a good option to control population excesses.
Remove the snails as you see them by hand or crush them against the glass. The fish will eat the crushed snails so there is no need to remove the food excess. Crushing, however, is not recommended with though snails as Melanoides snails (trumpet snails) and with large snails.
To facilitate the hand picking a bait can be used. Put some lettuce or potato pieces in the tank and dim the light for a few hours. Many snails are attracted by the bait and will collect on it, which makes it easier to remove the snails.
Another option is to use a 35mm film canister with holes in the lid and some bait like lettuce, fish food or bread in it. The snails will enter the baited trap and can be removed without much trouble.
The advantage of the canister method is that it enables you to use baits that strongly attract snails like fish food, without having the bait eaten by the fish. 

Complete tear down of the tank
This very drastic method is one of the best methods if you want to remove *all* snails from an aquarium, but for most of us, it the last option if all other methods fail.
A complete tear down means removal of the fish and the water, taking out all vegetation and collect the bottom material.
All fish should be put in a separated tank with adequate water quality. A good option is to use the water from the tank as the fish are already adapted to this water.
Remove all snails and eggs from the plants and after that they should be threatened with a weak potassium permanganate bath (see above) or another snail killing bath.
The gravel should be boiled to kill all snails and their eggs that are hiding in here.
Clean all other objects thoroughly and clean the tanks itself as well (don't use detergents for this!). A good sponge and warm water should do it.
The filter should be cleaned as well (obvious): put the filter without filter material in the 10 mg/l potassium permanganate solution for one hour (with the filter running). Rinse the filter with clean water and put in new filter material.
After all this the aquarium environment can be put together again.
Note!: The cleaned aquarium needs some time to rebalance again and the fish should only be reintroduced after this period (3 days to a week). Use a part of the old water to speed up the balancing.

Snail eating fish
The introduction of snail eating fish can help to control the snail population.
Not all snail eating fish are equally good in removing snails and some fish are great snails eaters, but aren't well suited for most aquaria. An example of the latter are puffer, which primary feed on snails, but also tend to get aggressive towards other fish.
A list of snail eating fish is available at the Ecology section of this site.

Chemical
There are many products available for extinguish snails from aquaria, but I won't recommend these as they are not healthy for the fish, plants and other creatures and the massive die off of the snails could create serious problems for the water quality.
If you go for this option, follow the instruction carefully and do regular water changes after the treatment. Check the ammonia levels during the treatment as these can rise to dangerous levels as a result of the decaying snail bodies.
Rather use the chemical warfare option if all other methods fail.

Rice and taro fields

Prevention

Clean planting material
It is preferable to use plants from area's that are known to be apple snail free. If that's not possible, examine the plants and make sure that there are no apple snails or other unwanted snails and also check for egg clutches.

Screens on water inlets
The use of a screen on water inlets helps to retard the spread of apple snails. Complete prevention of infestation of the field from adjacent fields is not likely.
The screen should be cleaned regularly to obstruction.

Barriers around fields
A barrier of copper could be used to slow the snail spreading to some extent. Copper is toxic to snails and they do not cross this material.
The copper wire or strip should be placed above the water level, on the border of the field.

Import regulations
Strict import regulations should be applied to prevent introduction of new species.

Controlling the population

Hand Picking
Hand picking is a successful method to control apple snail populations without harming the environment. The disadvantage is that it only works when done on regular base. The best results are obtained if the hand picking is done as a community effort.
All visible snails should be collected with a scoop net or by hand. After collection the snails can be used for human consumption (beware of parasites!), crushed to serve as a food source for fish or destroyed otherwise.
Eggs should not be forgotten during collection!
Baited traps filled with lettuce, cassava and taro leaves can be used to attract the snails and to facilitate the collection.

Bank Maintenance
All vegetation and obstacles around the field should be removed as much as possible as the snails need this to deposit their eggs. When there are no suitable eggs-laying sites available, the snails are forced to deposit the eggs on the bare ground where the eggs are very vulnerable and easily fall into the water, which drowns the eggs.

Trenching
Make some undeep trenches in the field before draining the field. The snails will congregate in these trenches and can be captured easily.

Predators and natural enemies
Introduction of apple snail predators should be carried out with great precaution if the predator is not a natural predator in the area as introduction of new species can have unexpected side effects that could worsen the situation. Nevertheless, the use of a natural enemy is a very interesting option. Ducks can be used to control snail populations as ducks can eat substantial amounts of snails, but they can serve as an intermediate host for trematode parasites, which can in turn harm the field workers. Another option is the Everglade kite (Rostrhamus sociabilis) for areas in Central and South America where this bird is endemic.
Recently, the fire ant Solenopsis geminata has been proposed as a possible biological agent against the apple snail Pomacea canaliculata.
A list of snail predators is available at the Ecology section of this site.

Chemical
-- under construction --

Killing a snail for dissection

Temperature
It is not easy to kill an apple snail without causing unwanted damage.
Using a chemical agent to sedate an (apple) snail can be difficult as the snails react to chemicals with a sudden retraction and closing of the shell. At such way, it can take a long time.
A often better and easier way to kill or sedate an apple snail is by placing the animal in the freezer at 0°C for about 12 hours (killing) respectively at about 5°C for 1/2 hour (sedation).
This method, of course, is most succesfull with tropical snails as they are unable to keep their metabolism at acceptable levels with low temperatures. Snails adapted to colder climats can resist this method much better. In such case, freezing the snail below 0°C can help to attain the same results.

Chemical
--

'Bibliography on the golden apple snail' section, 'Pest Alert' section and 'Asian distribution map' section.

Except where otherwise noted, this page is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License . http://www.applesnail.net