When is GFO exhausted?


New member
Typical GHA issues. Got behind on GFO changes over summer, and still not caught up. Until I can finally get an ATS going, it seems my only option is more GFO. How do I know how much, and when to change it. I've read it can exhaust in a few days or weeks.
I'm using a 1/2 cup, monthly, for a 75 in a BRS reactor.


New member
You can test the phosphate of the water coming out of your reactor. If it's the same ppm as the water going in then the gfo is exhausted.


Staff member
RC Mod
^Good idea.
It can deplete in days if the load is super-heavy when first starting. I make a practice of changing the medium once monthly until relief.


New member
Made the mistake of getting the wrong Hanna checker, not the ultra low one. Which kit do you recommend?


New member
I don't even check mine.. I just change the GFO once every two months. If im having a algae issue then once a month


New member
Just now I did some maintenance and much of the clumps of GHA came off with tweezers. I got about a half a cup (and maybe 50 asteria starfish). My WP25 on the right side was seized, after about a year, so there's limited flow on that side. I got on BRS and ordered an MP40 along with more GFO. It'll be nice to link it to the QD one on the left with the Apex!
Do you think that 1/4 cup every 2 weeks, instead of 1/2 cup each month makes a difference? I've always thought testing PO4 when there is GHA present is a total waste of time. But checking the outflow of the reactor is a great idea. At work, we use the term "minimum effective dose" and I'm still trying to figure out what that might be for my tank without going crazy. I could use a cup every week, but that would be pretty wasteful.


Premium Member
That is correct, the numbers themselves will not be representative of the total phosphate in the aquarium.

You want to manually remove as much algae as you can.