Very high phosphate numbers, but why

[Randy Holmes-Farley Verified Community Expert]

*UPDATE* So I have been maintaining the Lanthanum chloride dose over the last few weeks and my PO4 numbers were steadily dropping from 2.5 now at 0.9. After a weekly 20% water change the level of PO4 is 0.7, then increases over the next few days back up to 0.9. I can't seem to get the numbers to fall any further. I'm also adding Rowaphos in a filter sock. Feeding is strict, and I'm using a skimmer. Should I try something else, in addition to or instead of? or should I keep doing the current routine? Thanks

There is still a lot of phosphate bound to rock and sand, and it may take lanthanum to effectively bring it down in a cost effective way.