If you have these materials, this is a good way to clean it, as I had a 2x120 i flushed using these methods and it cleans your pump too. Take two buckets, preferably 5 gal, and almost fill one. Mix in some radiator flush (I know it needs heat to completely work but its probably better than tapwater alone). Take a length of tubing and stick it towards the bottom of the bucket, and connect it to the pump in and be sure to prime it so you dont ruin your pump. From the pump, go to the radiator and finally from the rad out to the empty bucket. This is the drainage unit and it will prevent messes and environmental contamination. Fire up the system (any 12v wall adapter will work if you dont want to hook up to your pc's supply) and let the whole bucket move through the circuit. When its almost empty fill with clean water and flush a little more. This ensures all has been removed from the radiator. If this makes sense, give it a try if you like and let me know if it helped as this method removed lots of trash from my system.