Since working on (and later building) my first water-cooled PC several years ago I have seen many different configurations from customers. Each of these required maintenance of some sort on a semi-regular basis, most commonly due to algae or corrosive buildup in the water loop. Other times a pump may have failed or a the occurrence of a blockage/leak.
This meant replacing faulty radiators, pumps, blocks or tubing. Depending on the complexity of the water loop(s) this could be a simple water replacement or a full system teardown.
The waterblock on the right is from my own PC’s CPU waterblock. The teal-coloured buildup is caused by plasticisers (or phthalates) leeching into the water from PVC tubing. This eventually built up in the narrow spaces between the cooling fins and caused a blockage. I solved this by cleaning the whole system and replacing the tubing with a high-quality phthalate-free option.