Figure 4. The concentration of soluble silica as a function of time after dosing. Dosing was done at t=0. The concentration for t=0 (30 uM) is an estimate based on what was added and the estimated total system volume.
Measuring Silica
Before going on to discuss silica in reef tanks, a few comments on measurements of silica seem worthwhile. In the context of organisms that use silica, we are only interested in soluble forms of silica, typically silicic acid and silicate. Ignoring the fact that there can actually be other soluble forms in certain situations, like extended chains or rings, the most important distinction that reef keepers need to be aware of is between tests that analyze for silicon, regardless of form, and those that analyze for soluble silica.
Tests that analyze for silicon, such as ICP (inductively coupled plasma) can include silica particulates (e.g., fine sand) in the result. Even with filtration, fine particles can evade removal. Consequently, it is complicated to extrapolate from an ICP measurement to a soluble silicate concentration. In Ron Shimek’s tests of
aquarium water,29 for example, the technique chosen was ICP. Consequently, people should not interpret the values obtained, 1.8 to 104 mM (0.05 to 2.9 ppm Si = 0.11 to 6.2 ppm SiO2 ) as necessarily indicating anything about the dissolved silica concentration present in the tanks studied (except that the dissolved silica cannot exceed those numbers).
Similarly, in studies of
salt mixes,30 the different forms may be an issue as well. In that case, the authors attributed differences between ICP and wet chemistry methods to the nature of the silicon present.
Tests that analyze for soluble silica, such as any of the kits available to the hobby, will only detect soluble forms. I recommend the low range silica test from Hach, Model SI-7, catalog number 22550-00. While the values obtained with such kits may not be comparable to those obtained by other methods, they are suitable for understanding how much dissolved silica is present and available to organisms in tank water, and in other aqueous solutions, such as tap water.
