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Salt water

Iron in the reef aquarium

by Akua Design 08 Jul 2021

In many reef aquariums, the only supplements added are calcium and a source of alkalinity (and all the chemical impurities that come with those additives and, of course, the food). No other, it seems, is required to keep many organisms.

There is, however, another supplement that has proven to be very useful in many reef aquariums: iron. Specifically, it has been found useful in applications involving reservoirs with substantial growth of macroalgae.
The advantage of iron appears to be at least double (and possibly triple). The main advantage is that at least some species of macroalgae grow faster and appear a darker and more attractive green, when the tank is dosed with iron. In addition to aesthetic benefits, this increased growth allows macroalgae to be a better nutrient export system. A secondary benefit is that faster growing macroalgae can compete better with microalgae, which is often a source of frustration for reef keepers. Another speculative advantage is that it can reduce the likelihood that the caulerpa will undergo sexual reproduction, creating water quality problems.


Iron in reef reservoirs: does supplementation make sense?

While iron can become a limiting nutrient in parts of the ocean when enough nitrogen and phosphorus are present, it makes sense that the same is true in our reservoirs, where nitrogen and phosphorus are typically present. in substantial excess over oceans. However, there is little published data on iron levels in reef aquaria, and even if overall iron concentrations were available for reef aquaria, one could still be misled if iron were not present under a readily bioavailable form (as in inorganic particles or strongly chelated by organic molecules).

Since the food delivered to reef aquariums contains a large amount of iron, how could the water column ever be "low" in iron? In the case of iron, there are several potentially important export mechanisms from reef reservoirs. Iron bound to organic molecules can be easily skimmed off, depending on the nature of the organic. Iron is also absorbed by the many organisms present in the reservoir. Additionally, iron in the water column may simply not be bioavailable when chelated to certain organics (as mentioned above and discussed in more detail below). Finally, iron can precipitate in any of the various environments found in a reef aquarium. These include high pH environments where certain additives are introduced (such as lime water), potentially causing the rapid formation of iron oxides and hydroxides. It is also possible for iron to bind to surfaces of calcium carbonate, both those present as sand and rock, and those created as coral skeletons and other biological structures.

Iron in Reef Reservoirs: When Could Supplementation Be Most Beneficial?

Let's start with the latter. In a reef aquarium without macroalgae, or without sufficient amounts for macroalgae to be considered an important sink for nutrients such as nitrogen and phosphorus, the addition of iron can actually exacerbate an existing microalgae problem. It could also tip the scales towards a microalgae problem if the iron restricts the growth of microalgae. In these cases, I would not add the iron or add it to stop the addition if the growth of the microalgae worsened.

The situations that could benefit from iron would be those where there is significant growth of macroalgae, with or without a microalgae "problem". In the case without any microalgae problem, the macroalgae can simply grow faster and thus reduce the levels of nutrients in the tank that are otherwise unwanted (such as phosphate which can inhibit calcification by the corals). The growth of macroalgae is, in fact, one of the best mechanisms for phosphate export in a reef aquarium, and optimizing this method can be of great benefit.

Iron in reef reservoirs: how much and what form?

Deciding how much iron to add is quite easy. Presumably, once you add enough to remove iron as a limiting nutrient, the extra iron apparently doesn't do any harm. On a dosage of 1 ml per 20 gallons of water. This liquid is dosed once a week in a system. The ideal is to use ferrous gluconate, which is much more stable. 

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