Susan wrote I don't think they're
oxygen deprived as I have never seen any of them gasping at the surface.

The thing that stood out from the OP was that they moved to the waterfall right
after sunset
which makes me think they are needing the oxygen.
If the OP is an early bird he/she could crawl out of bed just before dawn and
see where they are and what they do after the sun comes up.
Of course that's easier typed than done ;-)


kathy :-)
algae primer
http://hometown.aol.com/ka30p/myhomepage/garden.html