i had my tank going good for about 6 months or so. then the other day i noticed that i had 2 tiny snails on the gravel. i thought it was a miscolored gravel, since my gravel is blue. but i have never added any snails to my tank. i had these fish for 6 month and have not added any new ones. so how did these snails get in there? this is an inside tank so no other animals can get to it and i do not have any other pets, plus i always have the lid on it.