
Found this little guy in a pinch of aquarium algae. Best I can tell as a layman using https://nematode.unl.edu/nemakey.htm#38, this belongs to the genus Mononchus. They are predatory, consuming other microscopic animals including other nematodes, but apparently not parasitic (good news for my fish tank).
Wow. This is quite a giant in the world of nematodes.