Andy, No pics of my pond or koi.. Its not worth looking at
Their is a product that is made by bostick called chem caulk. This is basically polyurea in a tube so to speak. it is Moisture cured and also an adhesive. I would also use this on you bottom drain as well. Silicone works but his stuff works better and is fish safe. I believe is runs about $10-$15 a tube..
Maybe T, Zuber can explain further.. But this is what I would use...
Joe