Worms in my turtle tank?

I have a small snapping turtle and I a crappy filter so I clean it like once every week. I plan on getting a better filter soon though. Anyway, I noticed that whenever I clean his tank, there are these white little worms that are moving in there and they have black little heads, plus, there are these kinda wide worms and they are black, and I have no clue where they are coming from. Since the turtles tank is in the basement though, there are always these little gnats or other small flying bugs buzzing near the water surface, so could it be from them...? Oh, and last week after I just cleaned his tank, I noticed the tiny little white worms in the tank even after I clean all the turtles stuff off good. So what exactly are they and how do I get rid of them??