Question
We have just had a 14,000 gallon koi pond installed. We filled the pond in February and introduced our first Koi in the spring. Our koi are doing great but this year (I belive because of the extreme draught conditions we are experiencing in Georgia) there is an abundance of acorns falling from our numerous oak trees. My question is are acorns in a pond toxic or do they cause any other pond care issues? Thank you for any help.
P.S. I have found all of your previous answers very enlightening and your expertise is admired and appreciated.

Answer
That's a nice pond!  You should remove as many of the acorns as you can.  Like all vegetative matter, they will decompose in time.  Decomposition uses oxygen and releases toxic hydrogen sulfide and methane.  A few acorns won't do much but hundreds will.  Also, acorns, oak leaves, and oak branches being from oak trees may release lignins and tanins.  These are organic acids.  They may color the water brown and/or lower the pH.  The addition of fresh activated carbon to the filter will remove most of those.  You won't need to do that if you remove most of the acorns (and any other tree debris).  You should check the pH weekly while there is a lot of oak debris in the pond.  

Acorns are pretty heavy so it's probably easiest to use a pond net on a long pole to remove them versus a vacuum of any sort.  I assume you have an official koi pond which means it's bare bottomed?  If so, netting the acorns shouldn't be too hard.  A pond net with small enough holes will keep out most acorns.

Good luck!  Happy ponding!