Inconsistent Phosphates

[straightrazorguy]

I've been battling high phosphates ever since starting the current tank (18 months ago). I bought the whole setup from another reefer, who (I believe) neglected it for months. Not knowing any better, I used everything: his rock, his sand, etc. When I first started it, in Nov 2012, the phosphates were over 1 ppm. Over time I brought them down to below 0.1 ppm, by keeping to a rigorous water changing schedule and running ROWAphos.

What irritates me is the swings in phosphate levels that I noticed over time. Below is a table showing the results over the past couple of months. I test using a Hanna Colorimeter.
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DATE Phosphate Levels
13-Mar-14 0.15

15-Mar-14 0.17

16-Mar-14 0.11

18-Mar-14 0.1

21-Mar-14 0.08

25-Mar-14 0.12

27-Mar-14 0.13

30-Mar-14 0.13 Changed ROWAphos 2-Apr-14 0.31

3-Apr-14 0.08

6-Apr-14 0.04

10-Apr-14 0.06

15-Apr-14 0.09 Changed ROWAphos 17-Apr-14 0.3

20-Apr-14 0.1

22-Apr-14 0.3

24-Apr-14 0.1

29-Apr-14 0.06

30-Apr-14 0.05 After Shanging ROWAphos 4-May-14 0.06

7-May-14 0.48


I am totally puzzled. Any ideas/suggestions?