fish near waterfall
 

Should I be concerned? I have a 2500 gallon pond with water lillies and
about a dozen koi (18") and a dozen goldfish. Lately I've noticed the koi
are all hanging out under the waterfall. Nothing has changed - they never
used to do this. They eat fine, and otherwise no issues. Is it posible
they need more oxygen, or do they just like hanging out there?