<ol>
<li>A sudden drop in phosphate may stress certain organisms. This stress might be particularly important to corals with algal symbionts. The level of symbionts existing in a coral may depend to some extent on the availability of nutrients. A sudden drop in nutrients may imbalance the organism, leaving it with too many zooxanthellae for the newly-reduced nutrient levels. Especially if these corals are already living on the edge of survival, such stress may tip the balance toward disease.</li>
<li>In some cases, phosphate levels may drop below natural seawater levels, and phosphate may become the limiting nutrient. If this limitation is severe enough, corals and other organisms using phosphate may well be stressed, stop growing, and become more susceptible to disease.</li>
<li>Similar effects may result from a drop in certain trace metals. Since the effects of GFO on trace elements have not been clearly established in aquaria, it is possible that one or more critical elements may drop below optimal levels.</li>
<li>The release of soluble iron hydroxide itself may irritate certain corals, although many aquarists dose chelated iron without such effects. The iron hydroxide may, however, nucleate the precipitation of calcium carbonate in sub-optimal places, such as tissue surfaces. It may also bind directly to tissues.</li>
<li>The GFO may actually release certain metals other than iron from its surface. I have not seen any data on the chemical purity of these materials, and such issues may be a concern with some or all brands.</li>
<li>The drop in alkalinity and/or pH caused by abiotic precipitation of calcium carbonate would not be expected to be very great in most aquaria, and typically isn't especially large, as reported by the aquarists themselves. In the cases from which I've seen data, the effect is not as great as the variability between aquaria or between dosing events in many aquaria. Still, such changes might be important in some circumstances where conditions are already marginal.</li>
<li>Since GFO binds organic materials, the addition of a significant amount of fresh surface area may rapidly drop the dissolved organic levels. Such a drop may stress corals by rapidly increasing the available light levels, or by reducing a food source, or both.In order to minimize such difficulties, many aquarists start off using GFO more slowly than the directions might suggest. Such caution seems warranted in most cases.</li>
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