If you are just wanting to flush because you are draining and refilling and nothing funky is going on, then you can just drain/refill/drain/refill with some distilled water. If you want it cleaner, add some vinegar to the water to the loop, run the pump for a while, and drain/refill it with distilled until ALL of the vinegar mix is flushed out. IF you really want to flush it out big time, you can get adapters that are made for waterbeds from a drain/fill kit and hook up a hose from your sink into a barb on your computer loop, open up a good drain line, and crank on the hose. Then refill and drain with distilled and refill with your additives and distilled.