I would do a 100% WC every few days since they are poop machines.
It is a good idea to do regular water changes if you have many creatures in one tank, just like one would advise with fish, to maintain good water quality and to avoid accumulation of toxic substances and waste. By the way, apple snails are good indicators for the oxygen-level in your tank. When there isn't much oxygen in the water, the snails will regularly come to the surface to inhale fresh air through the breathing siphon
. Only when there is enough oxygen for them in the water, they don't need their lung
and solely depend on their gill
. The amount of water needed for each snail depends on the size of the snail, the filtration capacity and the temperature. As a general rule of thumb one should provide at least 10-liters/2.5 gallon for each mid-sized snail (±5cm/2inch. diameter). Or another way: count each apple snail as a fish of the same length as the snail's shell diameter. Be cautious not to overcrow your snail tank. While it hasn't been proven that apple snails actually 'sense' that they are in large numbers in a small space, the water quality quickly detoriates and substances that inhibit the snail growth are increasing quiker. Regular refreshment of a part of the water is highly recommended.
The water doesn't have to be deep (2 or more times the shell height). Remember that most apple snail species inhabit swamps in which the water level is quite low. Be sure to have a coverplate on your aquarium
or at least make sure that they can't get out of it or otherwise: don't be surprised to find a snail on the ground in the morning. Don't worry, they can easily survive out of the water for more then a week, but they risk damaging their shell
when hitting the ground.
If the tap-water in your area contains copper and/or other metals, use one of these water preparation products that catch away those metals bofere putting the snail in the water. Apple snails are very sensitive for these compounds (especially copper). You won't be the first one loosing a snail due to this snail-toxic substances in the water. If you see that the snails become completely inactive or if the snails, especially the little ones, try to leave the water after a water change: get a product to treat the water (like stress coat or Prime).
When there are many apple snails in a tank, the water tends to become cloudy because apple snails have a large amount of microorganisms in their intestine
, which help to digest the food, and which are expelled with the faeces. These micro-organisms (amoebocytes) should not cause any harm to the fish and can even serve as a food source for young fish. Fresh food (lettuce etc.) are more likely to induce this micro-organism based clouding of the water. If the snails are fed with dry fish food, the water will stay cleaner.
applesnail.net is a good place to go for everything you need to know about them