Agree with Mike. Also....
The daily dumping of the bottom drain; equates to a 10% change?
Aeration? Only the waterfall for this?
Is there any way you could plumb your current manual bottom drain to make it part of the filter circuit? ie. - Connect a vortex, then pipe to your barrels, then to your bead filter then finally to the waterfall? If this is plausible, you might want to consider a static filter in the first barrel followed by a moving bed in the second barrel, then use your bead filter to deal with the fines and slough off from the moving bed. (also, by using a moving bed your O2 would improve). Could be a fairly inexpensive improvement to your system.
Of course, the fish load is the easiest/cheapest fix for the immediate problem. Others here would be able to give you an accurate target for the number of fish you should have, but as an example, my main pond is 6K gals, and my goal is 6 fish total. (currently at 5, but anticipating a showa soon

)
Anyway, good luck.
Grant