All the typical PC water cooling coolants you can find are mostly distilled or deionised water mixed with various things like glycol and corrosion inhibitors, dyes etc.
Distilled water would be best, but deionised is fine. You can run purely with water if you wish but you’ll need to maintain the loop a little more often as otherwise you may get biological growth. The main advantage of the various coolants over just water is added protection from corrosion and they usually contain biocides to help prevent fouling.
If you are just topping up your loop a little then, assuming you are using a typical PC coolant, a little deionised or distilled water will have negligible impact on your coolant’s additive ratio. Add too much and you may make the biocides/corrosion inhibitors less effective and dilute any dyes but that’s all really.
Edit, ignore all that rambling - I totally missed which sub forum this was!