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.