I've a small planted tank setup with some gravel, java moss, and duckweed. Some weeks after I set it up, I noticed small white moving dots. Now, they look brown and they've overtaken the tank. They seem attracted to light. They swim around quite fast and clearly multiply at a crazy rate without fish to predate on them. I've a handful of shrimp in this setup and don't want shrimplets to be at risk. What could these things be?