What are the weird little things?
 

Currently my tank has base rock, live sand, a few hermits, some ceriths-who've done an awesome job clearing up the diatom bloom and 1 CB shrimp. Also some red grape macro algae.

Ceriths are evidently happy-eggs are laid every few days-hatch at 4 days( well assuming they hatch as its consistently 4 days)

I've noticed these tiny swimming things in the top few inches of my tank ( I'd take a pic but they are so tiny I doubt they would show up) they sort of float, then swim to the top, then drift down. Kind of remind me of mosquito larvae in their jerky swimming.

So are they the cerith larvae?
Do hermit crabs even breed in closed systems?
Or did I get an alien invasion?

Google either amphipods or copepods and see if they look familiar

They aren't copepods, had those since I added the macro algae about 3 weeks ago and they run around on the glass, the new floating things just showed up very recently-bout 4 days after first batch of cerith eggs hatched.