The goal of any do-it-yourself (DIY) recipe should be to allow safe and inexpensive supplementation without negatively impacting the seawater's chemistry. Aquarists can select either of two main materials to make such supplements, and three recipes are appropriate to make from these materials.
1. Epsom salts (USP grade magnesium sulfate heptahydrate) is readily available in drug stores and is fairly inexpensive. The problem is that if this were used to raise magnesium by a large amount (or a small amount several times) the aquarium water would become enriched in sulfate. This enrichment may not be a problem for some aquaria, especially those using salt mixes already deficient in sulfate, or those that employ frequent water changes. Nevertheless, Epsom salts alone is not an ideal magnesium supplement.
Table 4 shows the effect on an aquarium's sulfate level over time of using only Epsom salts to supplement magnesium, and when no water changes are employed. Clearly, the sulfate rises quite substantially over time. Figures 1-6 show the effect graphically with different water change amounts including 0%, 10%, 20% and 30% monthly, and 1% daily. In these cases, sulfate rises, but water changes can help mitigate the rise. If at least 30% is changed monthly, or 1% daily, using Epsom salts alone as a magnesium supplement may be acceptable; otherwise the sulfate buildup is likely too large to be optimal.
Table 4. Sulfate Rise Over the Course of a Year When Using Magnesium Sulfate to Supplement All Magnesium.
(Results are corrected for salinity changes.)
Daily Alkalinity (meq/L)
Daily Calcium (ppm)
Starting Sulfate (ppm)
Final Sulfate (ppm)
Sulfate Rise (%)
0.5
10
2710
3034
12%
1
20
2710
3353
24%
1.5
30
2710
3668
35%
[Note that these sulfate rise values are lower, for a given calcium and alkalinity dosage rate, in this model than I have shown previously for my DIY two-part additive recipe. The reason for this difference is that in that case, additional magnesium sulfate is required to offset the rise in sodium and chloride that comes along with the calcium chloride and baking soda. Such differences are unimportant for typical aquarists to understand, but this explanation is provided for those who noted and were confused by the difference.]
Figure1.jpg
Figure 1. The rise in sulfate over time in an aquarium with a daily dosage rate of 10 ppm calcium and 0.5 meq/L (1.4 dKH) alkalinity. This dosage rate is typical of an average mixed reef aquarium. Magnesium is assumed to be supplied by using Epsom salts alone. The various curves reflect different water changes scenarios, including none (black), 5% monthly (green), 10% monthly (yellow), 20% monthly (blue), 30% monthly (pink) and 1% daily (red).
Figure2.jpg
Figure 2. An expanded version of Figure 1, shown to provide a more accurate assessment of the relative rise of sulfate over the background concentration (2,710 ppm). The rise in sulfate over time in an aquarium with a daily dosage rate of 10 ppm calcium and 0.5 meq/L (1.4 dKH) alkalinity. Magnesium is assumed to be supplied by using Epsom salts alone. The various curves reflect different water changes scenarios, including none (black), 5% monthly (green), 10% monthly (yellow), 20% monthly (blue), 30% monthly (pink) and 1% daily (red).
Figure3.jpg
Figure 3. The rise in sulfate over time in an aquarium with a daily dosage rate of 20 ppm calcium and 1.0 meq/L (2.8 dKH) alkalinity. This dosage rate is typical of a reef aquarium with a medium to high calcification rate. Magnesium is assumed to be supplied by using Epsom salts alone. The various curves reflect different water changes scenarios, including none (black), 5% monthly (green), 10% monthly (yellow), 20% monthly (blue), 30% monthly (pink) and 1% daily (red).
Figure4.jpg
Figure 4. An expanded version of Figure 1, shown to provide a more accurate assessment of the relative rise of sulfate over the background concentration (2,710 ppm). The rise in sulfate over time in an aquarium with a daily dosage rate of 20 ppm calcium and 1.0 meq/L (2.8 dKH) alkalinity. Magnesium is assumed to be supplied by using Epsom salts alone. The various curves reflect different water changes scenarios, including none (black), 5% monthly (green), 10% monthly (yellow), 20% monthly (blue), 30% monthly (pink) and 1% daily (red).
Figure5.jpg
Figure 5. The rise in sulfate over time in an aquarium with a daily dosage rate of 30 ppm calcium and 1.5 meq/L (4.2 dKH) alkalinity. This dosage rate is typical of an small polyped stony coral-dominated aquarium with a high calcification rate. Magnesium is assumed to be supplied by using Epsom salts alone. The various curves reflect different water changes scenarios, including none (black), 5% monthly (green), 10% monthly (yellow), 20% monthly (blue), 30% monthly (pink) and 1% daily (red).
Figure6.jpg
Figure 6. An expanded version of Figure 1, shown to provide a more accurate assessment of the relative rise of sulfate over the background concentration (2,710 ppm). The rise in sulfate over time in an aquarium with a daily dosage rate of 30 ppm calcium and 1.5 meq/L (4.2 dKH) alkalinity. Magnesium is assumed to be supplied by using Epsom salts alone. The various curves reflect different water changes scenarios, including none (black), 5% monthly (green), 10% monthly (yellow), 20% monthly (blue), 30% monthly (pink) and 1% daily (red).
It's also possible to determine the effect on sulfate from one-time boosts to magnesium using Epsom salts. Epsom salts actually contain about four times as much sulfate as magnesium, so boosting magnesium gives an even larger boost to sulfate. Table 5 shows the effect on sulfate of several different increases in the magnesium level. From that table, it is clear that boosting magnesium by 100 ppm or more using Epsom salts has a substantial effect on the sulfate level. I therefore do not recommend using this method to boost magnesium by more than about 50-100 ppm. Every subsequent addition will compound the effect, although water changes will serve to reduce it over time.
Table 5. Sulfate Increase from a Single Magnesium Boost Using Magnesium Sulfate.
(Results are not corrected for salinity changes.)
Magnesium Boost (ppm)
Starting Sulfate (ppm)
Final Sulfate (ppm)
Sulfate Rise (%)
50
2710
2908
7%
100
2710
3105
15%
200
2710
3500
29%
300
2710
3895
44%
2. A second DIY material is magnesium chloride. Some grades of magnesium chloride traditionally have been contaminated in ways that would preclude their use in aquarium applications. They sometimes contain ammonia, for example. So any random magnesium chloride brand that's selected may not be acceptable. However, magnesium chloride hexahydrate from the Dead Sea Works seems to be adequately pure for this purpose. It is sold as a deicer (MAG flake from hardware stores, for example) or as a dust control agent for equestrian arenas.
Editors note (3/10/07): Note, the manufacturer of MAG flake has alerted us that they very strongly recommend against using this product in reef aquaria. While many reef aquarists have successfully used the product, the manufacturer does not claim to be able to provide this product at suitable quality in the future.
