rain, here's my own picture of 2mm plant worms.
most get these when their gravel gets too filthy.
fish don't seem to eat them, so you need to do
water changes and gravel vacuums to remove
and keep their population more unnoticeable.
here is an underwater picture of ianmason's dragonfly larva/pupa.
they probably come from aquatic plants grown outdoors.
you can manually remove or hope the fish will eat them.
rain- thanks again for inspiring this thread.
feel free to use this stuff in your main post.