ABSOLUTELY ESSENTIAL !
It is a plain fact that if you do not provide your Koi with the best water quality then, at best, you can not expect them to thrive and, at worst, they may end up ill or even dead.
To some extent, it is not important if your filter system is basic, the very latest model or where you source the water. But by the time your Koi are swimming in it the water MUST be of top quality. Testing the water regularly to ensure that it conforms to the following parameters is a good start.
pH - 7.4Ammonia - zeroNitrite - zero
Even after your filters are capable of providing water with these parameters there is still something indefinable which comes with a FULLY matured system and this may take a year or more to achieve.