Tap water isnt going to magically make your PC or watercooling loop explode. What will happen is, over months, it will discolour, grow strange green algae and look like fish poop in it after a while.
What is important is tap water is also conductive, meaning if your loop leaks, your PC is at risk.
Distilled / Deionised water (2L from halfords is probably less than a tenner) is better, although it should be changed after a year or so.
You can buy it in Tescos, its not going to bankrupt you. Its not conductive although you still shouldnt go splashing it around with the power on.
Proper watercooling coolant with anti fungal stuff and funky colours is best, and XSPC and Mayhems are well respected brands.
Whatever you use, buy a funnel, and have the kitchen roll close by.