How about chemical biocides? Killcoils are often debated as questionable when it comes to preventing microbial growth. Copper also has the same properties, just less per equivalent volume, but with the amount of copper in a loop vs. a killcoil, why wouldn't copper alone work just as well? However, copper alone does not really appear to prevent growth so there is argument that a killcoil alone likely wouldn't either. It's one of those tests that I'm wanting to run, but haven't really gotten around to it.
Many local super box retailers will carry some sort of biocide to prevent unwanted growth, or you can also check at your friendly neighborhood pet store. Most watercooling sites also carry growth inhibitors, but if the few dollars for a killcoil is too much, these cost around the same.