I noticed a fairly common but disturbing post on a pond forum today and I wanted to share this with you here in the hopes that it will help you avoid this with your Koi in particular.
Here’s the post
This is a story that would usually start with “A friend of mine…” but I’ll fess up. I made a major bone-head mistake and fed my koi while the water was still too cold. The weather was warm and they were obviously very hungry, but I didn’t think to check the water temperature. Now I have some very sick koi, including two that are near dead. They’re behaving strangly, some are losing scales, and a few even have blood streaks in their fins.
They’ve always been very healthy, and all of my goldfish appear unaffected, so I was baffled. After testing the water (all normal levels) and some serious googling I think it’s septicemia caused by the food rotting in their digestive tracts. I’m now trying to warm up the pond and running to the store for some salt. Any other recommendations for this emergency case? As always, thanks in advance!