Hello Mark . . .
We can keep our koi for many months in the temp range that Luther Chien identified as AA -- and never have an issue with ulcers.
It's not really about the bugs. It's not really about the temps.
It's about koi whose winter conditions were not properly managed, thereby stressing them out, degrading their immune systems and making them susceptible to parasites and bacterial infections they would otherwise shrug off.
Without that debilitating winter stress, those water temps shouldn't be a threat to our koi here in SoCal.
