Changing GFO if PO4 is 0.00?

[mpderksen]

My HANNA reads 0.00 for PO4. I last changed it at the beginning of April. I'm running a 1/2 cup in a BRS reactor with a 75 gallon tank. Testing weekly, I was waiting for it to rise, but it hasn't. Does this mean it's not exhausted yet and I can just leave it?
I don't want to be wasteful if it's still doing it's job, but I don't want to be cheap or lazy either.

[Crooked Reef]

It's not exhausted yet. I have had people say they replace it anywhere from two to eight weeks. Just wait until you need to change it unless it is collecting a bunch of detritus that may start breaking down to nitrates. If so either rinse it or replace it.

[MarksReef]

Mine too just hit 0 and is still good but I shut it off as 0 PO4's are not too good for corals.

[benjc]

Keep running it if you are reading zero on the Hanna AND you aren't seeing green algae growth in the tank. I can tell when I need to replace my GFO by a green tint that will start to form on my sand. Once the GFO is exhausted, algae growth will continue to keep my phosphate readings at zero on a Hanna for some time.

[Randy Holmes-Farley]

I agree that I'd judge by the tank and not just the number. If the tank is fine and the meter reads none (hopefully being accurate), then I wouldn't change it.

[mpderksen]

Thanks. I almost never need to clean the glass at this point. I had GHA, and that receding. My concern as stated above, is that the GHA is masking it, but I would assume that if the GHA is holding onto it, then it's not available for other bad stuff. Kind of like a heatsink. I use a PH to get stuff up into the column, blasting the crevices and any GHA. Then I take my reading.

[Randy Holmes-Farley]

Given that, I can't see a reason to change it at this point. :)

[AcroporAddict]

Given that, I can't see a reason to change it at this point. :)
Wouldn't the OP want to consider discontinuing GFO use until he has some measurable PO4 to remove? Maybe in the .02-.05 ppm range?

[Randy Holmes-Farley]

If it has already been in for an extended period and the tank seems fine, I doubt it is causing excessively low levels, despite the number, but removing it is also a fine plan. :)

[D-Nak]

While other people have had success with using GFO continuously, I use it only when I see a rise in phosphate (typically by a visual scan of the tank and I notice more algae than usual). My Hanna checker always registers zero. My assumption is that the chaeto in my fuge is absorbing it. I have a BRS dual media reactor -- carbon in one chamber and the second is either empty or is running GFO.

[AcroporAddict]

My tank is a 465 gallon BB SPS dominant, and I have continuously run GFO since I got into reefing in 2008. Not saying it wasn't always exhausted by the time I would change it, but a fair frequency for change out was about once a month.

I have read many postings where folks have said they felt their SPS did the best color was at a PO4 level between .02-.06. Anecdotal observation, but anecdotal observation from folks whose opinions I do respect.

I bought a Hanna ULR Phosphorus meter and my PO4 was very low, about .006 ppm, so I discontinued it and would like to maintain it at .04-.06 for a while and see how the corals do, and if I see any changes on growth and/or color for the better or worse. A couple days ago it was .049, so I am getting to the point where a further increase will have me wanting to use some GFO to lower the PO4 into the .04-.06 range, but my problem is I have never gauged the amount I have run, or measured the effluent output or whatever. All I have ever done is just fill the reactor up and change it out. Using the cylinder volume formula, I am guessing previously I was using about 5.5 cups at a time in the reactor and changing it out once per month.

Anyone with suggestions for how much to use for my purpose and how long the GFO will last until exhausted? I have BRS regular granular GFO, not the high capacity.

[Randy Holmes-Farley]

How much GFO and how long to wait to change it to target a specific phosphate level is too much of a function of how much you feed and what the other uptake mechanisms are to make useful suggestions (IMO). I'd go about it purely by trial and error.

[AcroporAddict]

How much GFO and how long to wait to change it to target a specific phosphate level is too much of a function of how much you feed and what the other uptake mechanisms are to make useful suggestions (IMO). I'd go about it purely by trial and error.

Randy,

Thank you for your help.

I use a 4" x 22" MRC up flow type media reactor that sits outside the tank. Do you think if I filled it with GFO as usual, then turned it on or off as needed, that would be OK? My main concern would be there would be some tank water sitting still for extended periods of time in the reactor when it was not in use. If I could do it this way, then I could just monitor tank PO4 levels, and when they start rising even when the reactor was going, it would let me know it was time to change out the GFO.

Mainly concerned the water in the GFO reactor would stagnate when the reactor is not in use. I have one sponge on the bottom of the reactor.

The amount of water in the media reactor would be 1.2 gallons minus the volume of the GFO in it. System volume is 450 net gallons.

[D-Nak]

Randy,

Thank you for your help.

I use a 4" x 22" MRC up flow type media reactor that sits outside the tank. Do you think if I filled it with GFO as usual, then turned it on or off as needed, that would be OK? My main concern would be there would be some tank water sitting still for extended periods of time in the reactor when it was not in use. If I could do it this way, then I could just monitor tank PO4 levels, and when they start rising even when the reactor was going, it would let me know it was time to change out the GFO.

Mainly concerned the water in the GFO reactor would stagnate when the reactor is not in use. I have one sponge on the bottom of the reactor.

The amount of water in the media reactor would be 1.2 gallons minus the volume of the GFO in it. System volume is 450 net gallons.

One of the reasons that BRS recommends that GFO should be placed in a reactor is because it needs to tumble. Otherwise it'll clump up. So, it really does need to run all the time.

[AcroporAddict]

One of the reasons that BRS recommends that GFO should be placed in a reactor is because it needs to tumble. Otherwise it'll clump up. So, it really does need to run all the time.
Not so much worried about clumping as I am about stagnant water. The GFO removes easily enough, clumped or not.

[mpderksen]

Update to OP: my reading today was 0.08, and the last change was 5 weeks ago. During the weekly maintenance yesterday, I was able to pull most of the remaining clumps of GHA out, I think due to adding AlgaeFix Marine twice/week for the last few weeks it is coming off easily. I also did my typical blasting of everything with a power head before the water change to clear any dead spots. My guess (and this is ONLY a guess) is that now that the GHA is nearly gone, it isn't sequestering the PO4 and I'll start getting more accurate readings. I'll change the GFO tomorrow, then test again this weekend.
The tank has been up since September, and I'm still monitoring it to learn it's personality. I have found that my Ca and Alk are steady with the dosing of B-Ionic 2-part for 3:30 min each night (~7mL). I've gone to testing that only every other week as a spot check, but I do it both before and after the water change until I'm confident.

Mg is still a bit of a mystery. But that's a separate post once I have enough data points.

Bottom line: 5 weeks for a 1/2 cup of GFO this time.

[lingeriesex]

I agree that I'd judge by the tank and not just the number. If the tank is fine and the meter reads none (hopefully being accurate), then I wouldn't change it.

[Randy Holmes-Farley]

Yes, I agree with the concern about it sitting stagnant and getting anaerobic. Mine does that it the cannister stops working for some reason, but it will depend on how long it is off, and the harm may depend on the nature of the other tank systems.

[AcroporAddict]

Yes, I agree with the concern about it sitting stagnant and getting anaerobic. Mine does that it the cannister stops working for some reason, but it will depend on how long it is off, and the harm may depend on the nature of the other tank systems.
Thanks Randy,
I won't let it sit in still water, then.

[Randy Holmes-Farley]

:thumbsup:

Happy Reefing! :)