Why does GFO decrease the alkalinity ?

ReeferGuyAlexK

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0.8 dKh is a large drop. Did you test more than once? What alk test were you using?
Yes i did. I use Salifert. Lets say it was test error but the amount of time it took to return to normal kh levels by upping the dosage with a about 5mls daily on my 250L system the math plays out to about 0.6dkh drop extra that particular day. Maybe it was due to the GFO and calcification of corals because of po4 drop.
 
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Randy Holmes-Farley

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Yes i did. I use Salifert. Lets say it was test error but the amount of time it took to return to normal kh levels by upping the dosage with a about 5mls daily on my 250L system the math plays out to about 0.6dkh drop extra that particular day. Maybe it was due to the GFO and calcification of corals because of po4 drop.

Yes, there certainly are indirect effects possible by reducing phosphate, but that effect should continue.

Do you rinse the GFO?

It is also possible that some types of GFO might be acid washed and may release some acid initially, but 0.6 dKH is a lot.
 

ReeferGuyAlexK

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Yes, there certainly are indirect effects possible by reducing phosphate, but that effect should continue.

Do you rinse the GFO?

It is also possible that some types of GFO might be acid washed and may release some acid initially, but 0.6 dKH is a lot.
I did rinse it yes but maybe not the best. I used Seachem PhosNet around 60grams for my 250L system
 

sgrosenb

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I, too, experienced a decent Alk drop when using GFO. You can see on my chart I started using it when Alk was at around 8.0-8.3, and it declined to roughly 7.3. I used about 2 cups of Bulk Reef Supply High Capacity GFO for my 165g tank (which I understand is a lot of GFO). I've noticed like @Randy Holmes-Farley said that it is most noticeable when the GFO is brand new for the first few days, and then Alk levels off. Maybe that means that the GFO should be changed out? Also - not to hijack the thread - but just curious how long it might reasonably take to pull all the PO4 out of the rocks, sand, etc before there are not many reserves left in the tank? I know it depends on a lot of factors, but a range of time would be helpful if possible, or some parameters on what might be realistic. Thanks!

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ScottB

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Glad I searched for this thread instead of posting yet another one. I guess this is by now just a PSA.

Perhaps my experience was magnified by the fact that my GFO bag was placed just downstream from my dosing lines, but Oh My Golly. Watch out folks. It also knocked down my calcium from 450 to 300 in about 10 days. (I rarely test Ca as it is always 450).

1642890940710.png
 
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Randy Holmes-Farley

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t? Also - not to hijack the thread - but just curious how long it might reasonably take to pull all the PO4 out of the rocks, sand, etc before there are not many reserves left in the tank? I know it depends on a lot of factors, but a range of time would be helpful if possible, or some parameters on what might be realistic. Thanks!

The rate of depletion will depend a lot on the level of phosphate in the water, and the flow through the GFO. Could be hours to months.
The effectiveness of the GFO in turns impacts how long it takes to remove phosphate from rock and sand surfaces, but that process can tank quite an extended period to get it out of low flow areas like pores in rock or deeper in sand.
 

ReeferGuyAlexK

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The rate of depletion will depend a lot on the level of phosphate in the water, and the flow through the GFO. Could be hours to months.
The effectiveness of the GFO in turns impacts how long it takes to remove phosphate from rock and sand surfaces, but that process can tank quite an extended period to get it out of low flow areas like pores in rock or deeper in sand.
How long does GFO media actually last? Seachem Phosnet in particular? Is it all dependent on the PO4 levels? Apparently it can be dried and used again, is that true?
 

Randy Holmes-Farley

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How long does GFO media actually last? Seachem Phosnet in particular? Is it all dependent on the PO4 levels? Apparently it can be dried and used again, is that true?

As I mentioned, it depends on the flow rate over it and the phosphate level. Higher flow and higher levels deplete it faster.

I think one needs to either figure it out in any given tank, or just guess when to replace it and figure out on your replacement rate, how much you need to attained your desired goal.
 

ScottB

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How long does GFO media actually last? Seachem Phosnet in particular? Is it all dependent on the PO4 levels? Apparently it can be dried and used again, is that true?
It is extremely common to have PO4 levels see-saw. New media will knock down the PO4 in the water and you get a "low" reading. A few days later, the PO4 leaches out from your substrate into the relatively depleted water, and your next test reads "high". Repeat

It is important (and annoying) to keep testing, as at some point the substrate will deplete to match the water. Your reef inhabitants by now have grown accustomed to elevated levels. Stripping the PO4 quickly and substantially will lead to trouble for the biome and corals. Fish don't care.
 

sgrosenb

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The rate of depletion will depend a lot on the level of phosphate in the water, and the flow through the GFO. Could be hours to months.
The effectiveness of the GFO in turns impacts how long it takes to remove phosphate from rock and sand surfaces, but that process can tank quite an extended period to get it out of low flow areas like pores in rock or deeper in sand.
I think one needs to either figure it out in any given tank, or just guess when to replace it and figure out on your replacement rate, how much you need to attained your desired goal.

This make total sense. Hoping my strategy is the right way to do @Randy Holmes-Farley - would be interested in any thoughts:

Over the past few months I brought my PO4 down from 0.20 to 0.02-0.06 via a GFO reactor. I check PO4 daily or every few days via Hanna ULR. When it gets down to about 0.02, I turn the reactor off for a few days and empty the media. After shutting the reactor off, it usually creeps back up to 0.05-0.06 in a few days. I replace the media (about 2 cups) and turn the reactor on again until it gets to 0.02 (usually takes a few days or a week).

Like I said I've been repeating this process for a few months now, making sure not to 0 out my PO4 in the process. Hoping it doesn't take too much longer to get to the point where it can stay at 0.02 - 0.03 without GFO, or at least with much less.

Thanks,
Scott
 
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Going to revive this thread with a question.. those who use enough gfo to notice a alk drop like myself (8.3 drops to 7.7) do you dose alk for the drop after changing the gfo or do you just let it come back over days from dosing? I was thinking of making the dosing adjustment after I change the gfo. It seems to drop within 2-4 hours. Next time I change the gfo I will test alk every 30 min to get a better idea of when the drops take place. Always striving for better stability. I don't notice any I'll effects on my SPS but maybe they could be better without this drop...
 

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