When you say you've used up a cup of ROWA in a couple days, what exactly do you mean? How did you come to this conclusion?
My experience with GFO has been to use smaller amounts and change it more frequently. IE: instead of BRS's suggestion of using 0.86 cups/225mls or so per 100g, I started running 100mls (thanks Mindy). I found this helped and feel it's due to not adding so much iron to the tank from the GFO.
I change my GFO every 4 weeks, and stir it every week or two to make sure it's still getting full contact.
Doing this, I can drop my PO4 from 0.09 to zero in no time.
I've dropped it from 0.19 to 0.05 in just over a week.
And if I run any more than the 100mls it will actually strip the tank clean.
Then I get other issues, so sometimes less is more
