Can anyone help me? mystery snails..?

i have 5 mystery snails in a tank and i am trying to breed them (unsuccessfully i might add). i have put a filter in the tank and i have a heater in there but i am worried that when i have (if i have) baby snails that they will get sucked up the filter.has anyone got any suggestions on how i can prevent this from happening. i have the water level lowered and set the heater on 27 cel. i have heard to put a stocking over the filter but wont this still suck them to the filter?