One of the first things my Koi mentor taught me was to be a good detective!
My mentor taught me that before I start treating, I need to ask good questions, and take the time to OBSERVE! What is the Koi's behavior telling you? What has changed? What are ALL the likely causes? Which of the likely causes can be ruled out? Take the time to insert your brain before doing anything! And of course, each rule has it's exceptions - if fish are dying rapidly, generally a large water change can buy you some time to find the cause (unless a water change caused the problem!). The CKK program (Certified Koi Keeper) here at K.O.I. teaches you the science behind all aspects of Koi keeping, so that you can learn to make good decisions. Problems happen! Learning to be a good observer, ask the right questions, and perform the correct treatments makes this hobby SO much more enjoyable! Why don't you try it? Have some FUN - Take one of the CKK courses, and if you like it and decide to take all 9 courses required for certification, K.O.I. will deduct the cost of that course from the cost of the CKK program! To learn more about the CKK program CLICK HERE.
As Spike Cover always says - "This hobby is a lot more fun with live fish than dead ones!"