Boiler system fluid

Tell us more about your system....
Boiler? radiation? tubing/ piping in the system?
Age?
Are you having issues?

Both are ok if you need them...
Pure de-ionized water would be the best. But you need the proper filter to fill and purge.

If it a new system some cleaner in for a week running around to flush out the oils would be good.
Then fill w/ clean DI/ de-mineralized water.
You shouldn't need the inhibitor.

http://axiomind.sasktelwebhosting.com/h2o-demineralizers.php

https://www.caleffi.com/usa/en-us/catalogue/filter-housing-na573100