Usage of GFO

[Walker]

I've been working on reducing my 90g tank's phosphates for a while now, and I got to thinking... when is it appropriate to change GFO? I'm working on the assumption that it's not good to zero out P04 and that it is ideal to have it in the .015-.03ppm range (5-10ppb).

 

I've been keeping records of my P04 levels, and I've figured out (roughly) that 1 cup of regular BRS GFO reduces my phosphates by 30ppb (.10ppm). So, my starting point of .22ppm took 2 cups of GFO.

 

Here's my conundrum. My feeding habits usually translate to an increase of 15ppb per week. I've gotten PO4 to .03 (good), but I know at the end of the week it will be at .08. If I replace my GFO though, my reef will net 0.0 PO4 for close to a week before it comes up to measurable phosphates.

 

How do people actually use GFO? Most guides online say to change GFO every 2-4 weeks, which means they'd be stripping phosphates for at least the first half of that duration. Isn't that problematic? Or am I missing something? Is there a lower limit to the phosphate-adsorption efficiency of GFO and it sits in a dynamic equilibrium even though it is not measurable? I'm just concerned as it is so easy to overdo it and it's a huge pain to get everything back on track.

[seabass]

You're right, you don't want to totally eliminate phosphate. I would try smaller amounts (changed out more frequently if necessary). You might also look at phosphate inputs, like how much/often you feed. Experiment and keep monitoring the levels. It might take a bit of work to dial it in.