Need tips for finding the source of Nutrients

[Proteus]

If space is a issue for a algea scrubber try using your overflow lines to supply water to the screen. Of coarse the would be on sided.
Or
Subman made one that's sumbmerced in his sump using a air stone at its base. Check his build thread

If your using a generic brand of gfo this could be a problem being you stated you change it one every two months. Once saturated it may release back into your water. This is why I use rowa. That say it dows not release once saturated. Also I use to remove any silicates.

As for your prodi bio I dose strontium and iodide seperate with the sea hem bottles at 5 ml every two weeks. I find it cheaper than the prodibio vials