I ditched my refugium when I went bioPellets and re built my sump with a larger reactor chamber to house more bioPellets instead
I had the same issue with BP and phosphates.. Could not get phos down low enough so had to install rowaphos. Now both nitrate and phos are zero and I like that I don't have to dose anything.
Different strokes for different folks. That's the great thing about this hobby. There are different methods to suit different people. Unfortunately this is also one of the most annoying aspects of this hobby! LOL