Hard GFO


New member
I know GFO will harden into a block when it absorbing phosphate...I also know increasing flow through reactor up to a point will keep it somewhat broken up...

I am testing with a hanna meter...reading zero for a month now...after 2 weeks my GFO (BRS GFO) becomes solidified...is it still usable if physically separated again or does that mean it needs to be replaced?


Randy Holmes-Farley

Reef Chemist
Premium Member
It is not from binding phosphate that it hardens, it is from calcium carbonate precipitating onto it. Breaking it up may help, but briefly dissolving the deposits in vinegar may be more useful (if you can do it without dissolving all the GFO).


New member
That was my problem that lead me to discontinue use of GFO. Dripping lime water and adding 2 part, that stuff was clumping up within a week. 'What a PIA to get apart and change. I would have to run muratic acid through it. And as Randy pointed out, it will break down into even more fines as it loosens up.

I'm now running a pro biotic system to control nutrients.


New member
I experienced the same thing. I switched to Seachem's Phosguard which isn't a GFO and haven't had a problem with clumping. It seems to remove the Po4 as well as GFO for me also.