I didn't notice these last summer, but these things just keep coming back. Not sure what they are called, but they swim under the water. Over the course of clearing the pool up, I noticed a few. I walked around the pool with the skimmer and got all that I seen. Now that my water is clear as glass, I seem them all over. I keep getting them out but it seems like they just keep on coming back. What are they and how do they keep getting in the water? And is there a way to kill them off?