I’ve heard of folks putting a small amount of Milton sterilisation fluid in with their water. I’ve also read criticism for this on other forums.
The best practice in my opinion is to start clean, use de-ionised / demineralised water / reverse osmosis (RO) / distilled water /ultra-pure water as best practice. This is essentially a water with nothing other than H2O in it.
Tap water will do though, in years of working with cutters, I’ve yet to see a calcium build up or slime actually affect a tube in a significant way. I’ve seen some pretty horrendous laser tubes in my time some black on the inside with slime, yet as long as the water was flowing, there was seemingly little significant issue with the tube. Obviously why take the risk? Most laser cutters have a flow sensor also, on the KH7050 this is a little plastic barrel, inline from the water-in on your cutter I’ve seen these damaged and fail in the “on” state from debris and slime in a water cooling system, so watch out for that.
Limit as much as possible exposure to light. I’ve seen water boxes and the hoses on chillers and coolers near windows and in conservatories go green with algae. Best thing is to regularly check your water, I do this by seeing if it reflects light back at me cleanly, if there is a dull oiling film of any sort on the water, change it out.
Let me know if this is helpful.