OK, as you can see it has darkened up as denitrifying bacteria naturally contained in the log consume nitrates and convert it into nitrogen gas that leaves the tank to atmosphere. I know now that this would never happen if I had boiled the log and sterilized it. Using feeder fish the first month to establish the log past the 1st hair fungus stage that the feeder fish love to eat. After that place in your ornamental fish, in this case Koi for now.
