Do not use EK nickel with silver coil. I know there is a lot of debate and science discussion online about wether it does or does not matter but I can say from first hand experience having nickel, copper and silver coil with deionized water completely ruined my EK blocks. Stick with all copper or don't use silver. I'm now running EK nickel with biocides and no silver now and have no issues
I had read similar stories about silver coils, so I didn't take the risk. I have nickel-plated and bare copper components in my loop and I use biocide in the coolant. I don't use a coil. So far I've not had any issues, but the loop is only 6 months old.