View Full Version : new GFO mess with ph?

11/01/2009, 08:29 PM
So know that I have a controller I am much more aware of things that are going on in my tank. So I changed my GFO today and noticed my PH plummet to 7.6 from 7.9. I assume this is normal, I think I recall GFO messing with ALK. Is there anything I can do to reduce this?

11/01/2009, 08:33 PM
7.6 is too low. Is the tank covered? Sounds like you need more gas exchange. If you open a window by the tank does the ph go up?

11/01/2009, 08:59 PM

It is usually 7.8-7.9. Chronically on the lower side due to a number of reasons. My tank is ok though.

I am more curious about the effects of GFO on the water chemistry.

11/01/2009, 09:41 PM
mine drops to 7.7 and skimmer stops skimming for about 3 days (bubbles do not go over the top to the collection cup).

11/01/2009, 10:36 PM
hmmmm, interesting.

Randy Holmes-Farley
11/02/2009, 07:03 AM
What brand? Some manufacturers may "pretreat" their GFO, if only to make it seem different than other brands. The pretretment may impact the pH of it.

In general, GFO doesn't seem to cause a big pH drop, but depending on what is binding, and if you are causing increased precipitation of CaCO3, you may get a pH drop from those effects.

I discuss some of those pH and alkalinity effects here:


What else might iron oxide hydroxide do? Precipitation of CaCO3

Many aquarists using GFO have reported unusually extensive precipitation of carbonates on the solid GFO, and elsewhere in the system. Such precipitation can, for example, be a contributing factor in the caking of such materials, and can coat other surfaces in the aquarium. This precipitation can also contribute to a drop in alkalinity and possibly pH as it removes carbonate from the water column. The effect of calcium will be similar, but smaller on a percentage basis, with a drop of only 20 ppm calcium for every 1 meq/L (2.8 dkH) drop in alkalinity. Increased calcification by corals and coralline algae (possibly spurred by reduced phosphate) can also cause similar drops in calcium, alkalinity, and pH.

Dissolution of these precipitates with acid, accompanied by bubbling, indicates that these deposits are carbonates, and are most likely calcium carbonate since it is supersaturated in most reef aquaria (and in the ocean). Several factors may contribute to this precipitation. Many of these are rather straightforward. It is known, for example, that phosphate inhibits the precipitation of calcium carbonate. Much like the role that magnesium plays in seawater, phosphate binds to the growing calcium carbonate crystals, poisoning their surface against further precipitation of calcium carbonate. Many organic materials are also known to inhibit this precipitation. Near the surface of the GFO, and downstream from it, the organics and phosphate are expected to be lower in concentration than upstream from it. The reduction in concentration of these inhibitors may well permit increased abiotic precipitation of calcium carbonate on such surfaces.

Two more esoteric events may, however, be equally important. The first is that the local pH near the GFO surfaces may be higher than in the bulk solution. This effect arises as phosphate and other inorganic and organic ions displace hydroxide from the surface. Figure 2, for example, shows phosphate displacing two hydroxide ions. The net swap of HPO4-- for 2 OH- will raise the local pH. The supersaturation of calcium carbonate increases as the pH rises, driving the precipitation of calcium carbonate.

Another possible role may be played by the iron itself. GFO is not completely insoluble. The solubility of iron hydroxide in natural seawater is small, but still significant (0.02 - 2 ppb), although it is largely controlled by the availability of organic ligands.11-13 One interesting possibility lies in the way that soluble iron actually impacts the precipitation of calcium carbonate.

At high concentrations, iron inhibits the precipitation of calcium carbonate. While different researchers find different threshold concentrations for this inhibition (>25 ppm in one case,14>7ppm in another case15), it is a well established and studied phenomenon. The mechanism is believed to be the same as for magnesium, phosphate, and organics, which all poison the growing calcium carbonate surface.

At much lower concentrations, however, iron actually increases the precipitation of calcium carbonate by acting as a site for nucleation of new crystals. In one case this happened at 100 ppb dissolved iron, increasing the rate of scaling (the precipitation of calcium carbonate on surfaces) by about 60%.14 In another case, the induction time for precipitation (that is, the time it takes for precipitation to begin once the water becomes supersaturated) was reduced by 40% at 1.4 ppm iron and the overall precipitation rate was increased by 32% at 560 ppb (lower iron levels were not tested).15 These studies were carried out in freshwater, and I have not seen similar studies in seawater.

Is the natural dissolution of GFO important in the nucleation of calcium carbonate precipitation? I am not sure. But it is clearly one possible explanation that fits the observations of aquarists as well as known phenomena involving iron.

11/02/2009, 09:13 AM
Thanks for the info RHF, I will have to read that a couple times to get in my head :)

I am currently using Dr foster smith GFO, but also have a bag of BRS GFO which I will use after the other runs out.

11/02/2009, 09:38 AM
Thank you for posting that!! It answered my gfo related ph questions :)

Randy Holmes-Farley
11/02/2009, 10:41 AM
Thanks, Folks.

I don't specifically know about any pretreatment to the Dr F&S and BRS materials, but I'd guess based on the type of business that they do not do anything to it.