Since this topic comes up over and over, I thought I give a summary of my current general thoughts on trace elements for reef aquariums.
1. First, a standard definition. Trace elements are those elements in seawater at very low concentration. It does not include the major ions of seawater: calcium, magnesium, alkalinity (carbonate and bicarbonate), sulfate, potassium, bromide, borate, strontium or fluoride, despite the fact that many commercial trace element supplements may contain some of these. The distinction is important in several ways that will become apparent in subsequent parts of this post, but I'll note here that each of the major ions of seawater are present in concentration above 1 ppm, while all of the other inorganic ions in natural seawater combined is less than 1 ppm total.
2. For major ions, the concentration does not vary by location or depth in the oceans. The only significant variation in major ion concentration comes as the salinity changes. Trace elements, however, are different, and can vary considerably by location and depth. Some are surface depleted. Some are depleted deeper down. No single number, for example, can tell you the natural ocean concentration of, say, iron. If one is targeting a “natural” concentration of iron, what number would one choose? The ocean does not tell us a definitive answer.
3. All organisms need a number of trace elements for a wide range of biochemical processes. These include iron, copper, zinc, manganese, vanadium and molybdenum. Some trace elements are purely a toxicity concern, including mercury, lead, and cadmium. Many are needed at one concentration and are toxic at higher concentration (e.g., copper and nickel). Organisms, such as fish, likely get some or all these needed trace elements from foods rather than from the water itself, but many organisms do get them from the water, and all organisms that do not consume particulate foods in some fashion must do so.
4. For organisms that do get their needed trace elements from the water, there is very little experimental evidence on how much is too little and how much is too much and might be toxic for any given organism. There is a fair amount of experimental evidence in reef aquaria about how much of many trace elements in the typical forms found in reef tanks is “adequate” for the organisms, especially corals, but not really what the acceptable range is. Some of the ICP-based trace element methods use this adequateness approach. In general, reefers have found that the acceptable levels of some trace elements can vary a lot more than the acceptable levels of some major ions. Iron, for example, seems to be able to be acceptable over a very wide range of concentrations (certainly more than a factor of 100) and still be adequately available and not toxic.
5. For major ions, the concentration, and perhaps pH, tells you all you need to know about its bioavailability. 420 ppm calcium is equally bioavailable in every reef tank. Many trace elements, however, can exist in a variety of different chemical forms. These differences include different oxidation states, such as ferric (Fe+++) and ferrous (Fe++) iron. They can also include different complexation by organics. Copper, for example, is known to be nearly entirely bound by organics in the ocean, and that binding greatly impacts (reduces relative to the bare ion) its toxicity and bioavailability. Thus, the concentration of a trace element (such as by any type of ICP) may only provide a part of the question of whether there is enough or too much or too little of a trace element present.
6. The oxidation state and the complexation by organics can change rapidly in a reef aquarium. Thus, the form one doses may immediately change to something else when mixed into the water, and may also change as it experiences various treatments, such as ozone, UV, hydrogen peroxide, antioxidants, processing by organisms, etc.
7. The depletion of trace elements arises in several ways, including uptake by organisms (corals, anemones, algae, bacteria, etc.), binding to mineral surfaces (calcium carbonate, GFO, etc.), and through any sort of organic export mechanism (skimming, activated carbon, polymer resin absorbents, and physical filtering of “detritus”). Many reefers assume that fast growing SPS corals are the driving force behind trace element depletion in their aquaria, but IMO there is little evidence of this. When folks use methods such as macroalgae or turf algae to control nutrients, organic carbon dosing to drive bacteria, skimmers and GAC to export organics, or even particulate calcium carbonate dosing to keep the water clear, these may be equally large or larger sinks for trace elements.
8. Some trace elements have been found to rapidly deplete. These include iron and manganese. They can drop from dosed levels to undetectable by typical hobby testing in a few days. A small amount of macroalgae growth can strip a whole tank of manganese. Some can be much slower to deplete (e.g., zinc). If one chooses to just test the waters of trace element dosing, iron and manganese are a good place to start. There are both DIY and commercial products for just these, and many people have found them useful.
9. Folks thinking about consumption of trace elements in reef tanks often think about water changes as the way they are replaced, and it is true that new trace elements come in with water changes. However, there are additional factors that bear on reef husbandry and our interpretation of the usefulness of our actions.
A. Rapidly depleting trace elements cannot ever be maintained at the concentration in the salt mix by water changes alone unless one changes 100% of the water every day. However, some salt mixes may have more than natural levels of some trace elements, and since the acceptable level of a trace element may be well below that present in the salt mix, water changes may be useful in adding trace elements.
B. A widely ignored source of trace elements may actually be the primary way many trace elements get into reef aquaria. Foods are loaded with trace elements, for the same reason that organisms need to take them up: all organisms and hence all foods sources must contain them. For some, the total amount of certain trace elements (such as iron) may be far higher in daily foods than in daily 100% water changes. However, there are no studies that show how well these food-contained trace elements get into and become part of the food chain in a reef tank. Certainly some is lost, but my expectation is that a substantial amount of trace elements do get into the water this way.
10. Many folks dose trace elements to try to replace those lost in the aquarium, and there are many commercial mixes and DIY recipes. Deciding how much of what to dose is a vexing problem that may be best answered by trial and error (which successfully deals with all of the uncertainties described above) but it takes a lot of time and effort. Folks attempt to shortchange that effort, with a number of different methods that try to eliminate some of the uncertainties, and I’ll describe the pros and cons of these below.
A. Some commercial trace element mixes are designed to be used in a volume dosed per day or week methodology. For example: Add 1 ml of solution to each 100 L of aquarium water daily. Certainly the easiest way for the reefkeeper, but they can only be “perfect” for a single type of reef tank. That said, they may be adequate for a reasonably wide range of reef tanks. A beginning reefer might start here with an additive from a company they have confidence in, since the reefkeeper is fully trusting them to get it right, and IMO, not all companies have earned such trust. One might consider experimenting with lower or higher doses over time to better match the actual needs of your aquarium, and might start high or low if there is more or less growth in general in the aquarium relative to an average tank. A new reef tank with few organisms will certainly take up fewer trace elements, and more is not necessarily better.
B. A second approach ties the amount of trace elements added to the calcification rate. Say, to alkalinity demand per day or calcium demand per day. For example: Add 1ml of supplement for every 20ppm of calcium added per 100 liters of aquarium water. The company makes some sort of determination of the amount of trace elements needed per unit of calcification for a typical reef tank. A number of products do this either explicitly (for a trace supplement) or implicitly, such as with a two part or one part alkalinity and calcium method that has extra added trace elements.
The calcification rate would be
