We have a laser system similiar to HERL station. Even though we use purified water we have to clean the cooling circuit of our laser every few month. We will start to monitor the water flow in the tubes to see if too much deposits are reducing the flow and with that the cooling as well as the laser power.
We were talking to a technician from the company who said that the inside of the laser the pipes are made of Aluminium which probably reacts with the purified water ... generating white deposits that start to collect and block the pipes. Not much we can do about it except monitoring and cleaning. Descaler helps to remove the deposits quite well.