I build a few water-cooled systems in my life and I know where you're coming from but this is a bit different, instead of fins a heat pump uses a plate heat exchanger and all pipes are at least 22mm wide, a thin bacteria film would do basically nothing in the grand scheme also because it's desalted water there are just a few bacteria colonies left that gone die in an "airless" (in quation because it's in an engineering sense airless not in an physical sense) system without any food in a few weeks to days. Also an anti microbial agent could be aggressive against some materials in the loop and one that isn't should be rather expensive. Especially in that volume of water. We're talking about an office building with an attached workshop. Together with the buffers it should add up to 3000l of water in circulation. Having roughly 10% of an agent that is probably expensive and really doesn't do anything would be not that efficient.

TL:Dr: cost/efficiency factor isn't there to begin with