Yes, I believe there is a concern about driving phosphate down too low. I have an article on them coming out in a week or so. From it:
"Suggestions for using iron oxide hydroxide to bind phosphate
I have not used (in an aquarium) any of the commercial GFO brands sold to aquarists, although I have examined them in various ways. There are obvious differences between them, including the amount of water present. Rowaphos, for example, comes as a wet solid while Salifertââ"šÂ¬Ã¢"žÂ¢s product, Phosphate Killer, comes as an apparently dry solid. Other differences could include the surface area, any surface chemical treatments, the amount of fine particles present, and other factors. For these reasons, I should stress that one should not assume that they are all the same. I am not prepared, however, to make any claims about the relative efficacy of the various brands.
There are published experiments that purport to examine the relative effectiveness of different brands. In one such study, the phosphate binding data was largely generated at phosphate levels above the range likely to be encountered by most reef aquarists (0.5 ââ"šÂ¬Ã¢â‚¬Å“ 3.5 ppm), while at normal aquarium phosphate concentrations (up to about 0.2 ppm), the two products appeared equivalent. In a second study, two products being compared were dosed at high enough levels that both products being tested bound nearly all of the available phosphate (down to 0.01 ppm). Such a test is akin to comparing the pain reducing efficacy of morphine and aspirin by showing that both relieved a headache. A possibly better test would involve dosing less and seeing which brand (if either) lowers phosphate the most.
For aquarists that choose to use such materials, they should be aware of the possible biological problems that other aquarists have encountered. Starting slowly and allowing the phosphate to decline over a period of a week or two may be less stressful than dropping it in a period of hours, regardless of the mechanism of the problems encountered. Using a smaller amount of material, and changing it more frequently may also be less stressful. Salifert recommends using 250 mL (8.5 ounces) of their product (Phosphate Killer) to treat a 125-250 gallon tank for up to three months. However, there is nothing wrong with starting with 1/10 that amount to see what happens. While it may be more work, using 1 ounce and changing it after 2 weeks may reduce some of the issues that aquarists have observed on changing the media.
Aquarists should also be aware that dropping phosphate to extremely low (growth limiting) levels may bring on undesirable effects that reef aquarists do not typically encounter in the absence of using such materials. It is quite possible that if GFO can bind enough phosphate to limit the growth of algae, that it may be possible bind enough phosphate to limit the growth of other organisms, such as corals. Iââ"šÂ¬Ã¢"žÂ¢d suggest using a phosphate test kit as a guide to how much material to use (i.e., use more if you maintain levels above 0.02-0.03 ppm, and perhaps use less if you never detect any phosphate). Kits can also be used to determine when to change it (change it if phosphate levels begin to rise after a period of being lower).
Finally, be sure to rinse these materials in fresh or salt water before adding them to the aquarium as fine particles may get loose in the aquarium, clouding and coloring the water, and possibly creating other problems. There is no efficiency drawback to this rinsing. For aquarists that are using the GFO in a fluidized bed reactor or canister filter, just run it on some water change water for a few minutes before putting it into the tank. A media bag of GFO can just be rinsed with salt or RO/DI water a few times before adding it to the aquarium. Do not squeeze the GFO inside of the bag when rinsing as that may break the particles into smaller bits that can then escape the bag.
The bottom line: Would I use GFO to export phosphate? The answer is a qualified yes. Phosphate is such a significant problem for reef aquaria that it should be kept appropriately low (less than 0.03 ppm) in some fashion. There are many methods to minimize the accumulation of phosphate. For my own reef, I prefer to use skimming, carbon, and macroalgae growth to export phosphorus. If these are unsuited to a particular setting, then perhaps GFO is an appropriate alternative."
- thanks Randy.
