Good Biocide - Will last for years... 
Written By Peter Baldry on Mon 30th Mar 15, 14:14This biocide works well. Been in my loop (distilled water) for a few months now and keeps it from "growing anything". Only a couple of drops per litre so will last me for a while. Plan to drain and refill every 6 months.
https://www.watercoolinguk.co.uk/p/Liquidcool-Nuke-CU-Concentrated-Biocide-10ml_20876.html20876Liquid.cool Nuke CU Concentrated Biocide - 10mlhttps://images.watercoolinguk.co.uk/images/product_images/main/307/liquidcool-pm-nuke-concentrated-biocide-10ml-lc-liq001-20876-1.jpg3.98GBPInStockWater Cooling/Accessories/<strong>Latest addition to the Liquid.cool range</strong><br /><br /><strong>Warning:</strong> This product contains Copper sulfate. <strong> DO NOT INGEST!</strong> AVOID CONTACT WITH EYES AND SKIN! KEEP OUT OF REACH OF CHILDREN AND PETS!Liquid.coolstock_yes5
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