Fernox protector. Commercial grade inhibitor. At the end of the day the stuff you're buying online is just inhibitor marked up massively. Algae only grows when water lies dormant for long periods of time. My old server hasn't been turned on in 5 years. I used Canadian Tire glycol mixed with water. It still has no growth. Never had an issue. I should fire it up and see if it still works.
My last custom water loop had nothing added but it ran 24/7 and never grew anything either.
The fernox is about 80 bucks for 500ml and would do your next 20 loops with tap water.
Corrosion inhibitors, otherwise known as Corrosion Protectors or central heating inhibitors, safeguard central heating systems against the formation of sludge and scale build–up. The damaging effects of corrosion debris can cause boiler noise, radiator or pipework blockages and boiler breakdown...
fernox.com