I agree no point in trying to band aid fix a phosphate problem until you know the cause. I had high levels in my 90g and was advised to get rid of some of my sand bed depth as I had 2-3", so I removed a 5g pail and a half of real dirty sand bed and am now at 1" max. The detris build up was causing some of my problems, as well I added another wp25 wavemaker power head to increase the flow in and around the live rock which helps to blow any build up off them.
I have about 100 gallons of water volume between the DT and the sump after taking into account rock displacement. and do 25% RO water changes every 2 weeks. I also run GFO which I plan on changing every 4 weeks, until my level is at an acceptable reading, once it's there I'll check the PO4 level every couple of weeks and when I see the levels climbing I'll know it's time for a GFO change. I was using a color chart PO4 tester but find the Hanna checker is the most accurate for phosphates and use Salifert kits for everything else.
This is what I did for my system and it seems to be working for me so far, hope this helps :smile:

phos
 

use precision solutions phosphate solution to bring it down and gfo to keep it stable

Foz Down
 

Hi,

The Foz Down will work very well for your system. It is safe for all the inhabitants of your tank and a lot easier to use than GFO or Rowaphos. Once I started using Foz Down I threw the expensive High Capacity GFO and the PITA reactor to the curb. ;-)

Cheers,
Tim