I set up this 20gallon tank about a month or so ago and I've been letting it run with a filter and doing water changes here and there. I am planning on getting fish to put in there to reproduce. But I noticed the other night that there were 3 snails crawling around in there! I haven't bought any snails nor have had any life besides a plant in this tank! Can someone please help me understand how these little fellas got in here?