I soaked my Koralia's in a 3:1 vinegar to water mixture and they came out looking like new. Once they are done soaking, get a bristle brush and scrub them in water.
I also soak them in a vinegar-water solution, then I scrub them with a bottle brush. The bottle brush gets into the slits in the housing so it does well.
I usually drop the magnet on the pro in a cup of full strength vinegar then pour some in the hole.The rest I use an old hard bristled tooth brush and hot water.If their real caked up with algae,i'll give them a few minutes in 1/2 and 1/2 bleach/water.