I used pH Neutral from Seachem for months before I started to keep a planted tank. By the time I tested for phosphate, I got a dark blue reading right away when I added the indicators. My phosphate was very well over 10! O did water changes almost every days for a few weeks and I didn't get a reduction. AFter reading, I found people suggesting doing deep gravel vacs to get the phosphate out of the substrate. I did this also, but finally got some PhosGuard and changed it religiously every few days. This got my phosphate levels back to normal.
I'd echo what everyone here says as far as not using chemicals when you don't have to. Try the gravel vacs a few times and if nothing changes, it's probably in your substrate and leaching out as it can. You may need to use the PhosGuard if other methods don't work.
30g planted with cories, white clouds, Harlequin Rasboras ,ottos.
10g planted with glowlight tetras and an otto.
Outdoor pond with one common goldfish and comets.
5.5g with endlers.