Excellent water is much more than just clear looking water. Water to koi is like air is to humans. You can be surrounded by air that is perfectly clear and yet so full of toxic fumes that it can be life threatening. The same is true with koi. Clear water is no indication it is healthy. Only by testing your water can you know what condition your pond is in. These test kits will allow you to to do just that.
This specific test is for Nitrite.