I have a 15 gallon tank with guppies in it. Tonight I was looking at all my tanks and noticed something I have never seen before. My 15 gallon has what looks to be like thousands of tiny brown dots floating around and down on the rock in there. I know they are something that is alive by watching them. The only thing I have ever had in this tank was an oranda and 6 ghost shrimp before I changed it to a guppy tank. Can anyone give me advice on what these might be and where they came from?