IMO, diatom filters are over-rated. yea, they work great temporarily, but the greenwater comes right back. i've used one before and just was NOT pleased with it. 
i then bought a UV sterilizer. WHAT AN INVESTMENT!!! it works SO much better than the diatom filter. i think the problem with diatoms is that they don't filter out every single particle or whatever that is causing the greenwater. with a UV sterilizer, you can see it working and you know it is killing the evil little algae guys that are making the water green! i recommend that EVERYONE with a canister filter get one. 
i then bought a UV sterilizer. WHAT AN INVESTMENT!!! it works SO much better than the diatom filter. i think the problem with diatoms is that they don't filter out every single particle or whatever that is causing the greenwater. with a UV sterilizer, you can see it working and you know it is killing the evil little algae guys that are making the water green! i recommend that EVERYONE with a canister filter get one. 