General Ecology Europe 
Spark-L-Pure® Micro Water Filter
Koi keepers happily buy sophisticated filter systems for their ponds but many still don't bother to install effective water filtration systems. Water companies may well claim to provide tap water suitable for human consumption but they take no such responsibility for Koi.
Chlorine, Aluminum Flocculates, Pesticides and Herbicides may well be ok for humans - but they can mean big trouble for Koi!
Placing them carefully in your NI design Koi system with state-of-the-art filtration system and then ruining them by putting untreated water containing contaminants is an avoidable error. While untreated water might not kill Koi, it may kill their potential. And, water varies in quality over time. Water that is fine today may not be fine tomorrow or the day after.