Sean wrote This only seems to happen in the early
evening, when the direct sun is gone -

After the sun goes down plants stop making oxygen and start using it up. The
waterfall may be providing the extra oxygen the fish need to get through the
night.
Your fish have grown and probably mulitiplied so it may be that this particular
fish load needs to use the oxygen provided by the waterfall where they didn't
in previous years.

I saw this happen in one of my ponds when I added a load of azolla which spread
over the pond. By morning, before sunrise, all the fish were at the spitter and
also gasping at the surface. As soon as the sun came up they took off. I
removed a lot of the fish and the behavior stopped.




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