So, I do not have an easy access to premixed fluids so I wanted to use distilled water but I still don't know what type of biocide I should use and if the quality of the distilled water matters etc. If someone could avise i'd be extremely grateful.
I use plain steam distilled from the local grocery store. Been using silver coils since 2013 or so. I drain and fill my loop every 6-7 months and clean blocks every 1.5 yrs or so. Even today, the argument still continues on "best" coolant options and safest. There are some biocide drops, was PTNuke back a few years. Would have to look around to so what's common today for non-premix options.
Distilled is great, but yes, you need anti-microbial to prevent crawlies. There are some decent 'clear' coolants that are good, prevent growth and don't have the gunk in them that can cause you problems like clumping or fallout.
Distilled water for me (less than $1 a gallon at any supermarket) and have used PTNuke, copper sulfate and even drops to prevent fish tank algae. I've seen a lot of back and forth on how much a KillCoil actually does...and how copper also naturally should act similarly.
Might be something I could test - I have plenty of extra hardware laying around and could run a loop 24x7 in the sunlight to see what happens.