I've had a reef tank for 2 years now and water quality stays great except for the phosphate. I added phosgaurd to the sump and it didn't drop after about 2 weeks, so I hooked up a canister filter and put the phosgaurd in it since it moves much more water than the sump does and still it will not go down. Right now I'm showing .5ppm phosphate. How long should it take for phosphate to go down. I do use a rodi filter and have between 0-1 DS. Thanks for any advice.