Iron Oxide Hydroxide vs Magnesium

[reeffantic]

If Iron Oxide Hydroxide is so good at binding with metals, will that not affect the magnesium level in your tank too?

[Randy Holmes-Farley]

I presume that you are referring to this comment that I made in my GFO article:

Iron Oxide Hydroxide (GFO) Phosphate Binders
http://reefkeeping.com/issues/2004-11/rhf/index.htm

"What else does iron oxide hydroxide bind? Metals

These materials are known to bind a wide range of other compounds from water, including trace metals, arsenic, selenium, silicate, and organics. Metals such as manganese, cobalt, nickel, and zinc are known to bind to iron oxide hydroxide in simulated seawater solutions. It has also been claimed that the binding of copper and zinc by natural iron oxide hydroxide sediments exerts a powerful control on the concentration of copper and zinc in polluted rivers and estuaries. Although not studied in seawater, it has also been observed that phosphate binding by iron oxide hydroxide actually increases its binding of copper, cadmium, and nickel in freshwater.

Whether the binding of any of these ions is important in aquaria, and whether it should be considered a benefit or a detriment, remains to be established for each trace metal. Nevertheless, it is something that aquarists should keep in mind, and it may also be important in suggesting potential explanations for some of the biological effects of using these materials that are discussed later in this article.
"


I've not seen any data on whether it binds magnesium. However, there is so much magnesium in seawater that there is little chance that reasonable doses of iron oxide/hydroxide will have any direct effect on the magnesium concentration. In a 100 gallon tank there is more than a pound of magnesium ion. It would take a huge amount of GFO to bind that much magnesium even if it bound it as strongly as phosphate (which I do not believe it will).

For comparison, at a phosphate concentration of 0.1 ppm, there are 54,000 times as many magnesium ions in seawater, as there are phosphate ions.