Pond snails can be a big problem for your water quality if they overpopulate... They have a giant bioload so you will need to perform more and more water changes the more they reproduce. Since they are asexual they also reproduce faster than rabbits... You might want to remove some occasionally, especially the larger breeding sized ones as well as any eggs sacs you see. If you don't there will be hundreds before you know it. Once of my tanks got to the point where the entire floor was snails and dead snail shells. Not pretty or healthy.
Nodmas113- you must have Mystery Snails (aka Apple snails) they will reproduce but they lay gigantic eggs sacs at the surface of the water that are really easy to notice and remove. They don't reproduce often so it isn't usually a problem. They have a large bioload though so careful with how many you keep to a tank.
Last edited by DarkMoon17; 06-08-2011 at 11:19 PM.