I was cleaning my freshwater aquarium gravel and have found loads of black conical shelled snails in the gravel ... I have 4 apple snails but these are nothing like them - the biggest one is about 1 cm long and the shell is a long thin cone very dark almost black - i've since noticed they hide when the light is on burying themselves in the gravel but come out when the light is off .... any idea what kind of snail it is? looks like I've at least 30 of the things but I can't treat the water to kill them without harming the Apple ones.