Do you rinse the frozen food with RO? I'm sure that the liquid contains extra phosphate which just goes straight into the water.
Is the tank less than a year old and did you initially have issues with low phosphate?
As as been mentioned above, phosphate will get absorbed by the rocks. What can initially appear as a low phosphate issue can quickly flip and become an excess as the rocks become saturated. It's common in newer tanks (mine did it) especially if you work hard to increase phosphate only to realise you've fuelled the impending problem!
I use GFO (Rowaphos) in a small reactor which I luckily have room for in my sump. I initially tried it in a bag in a high flow area but it just clumped together and didn't really work.
Is the tank less than a year old and did you initially have issues with low phosphate?
As as been mentioned above, phosphate will get absorbed by the rocks. What can initially appear as a low phosphate issue can quickly flip and become an excess as the rocks become saturated. It's common in newer tanks (mine did it) especially if you work hard to increase phosphate only to realise you've fuelled the impending problem!
I use GFO (Rowaphos) in a small reactor which I luckily have room for in my sump. I initially tried it in a bag in a high flow area but it just clumped together and didn't really work.