Strontium in that Seachem product is too low to match demand with calcium. It has a fixed ratio of Sr to Ca in calcium carbonate which will only vary with the Sr/Ca ratio in the water, and not so much by how it is deposited (which coral, abiotic, coralline algae, etc.).
Magnesium is more complicated. It varies with the way it is deposited, and is a lot more in coralline algae than in some corals.
For DIY, there are two reasons to add magnesium, but the incorporation rate involves a big assumption of how much is needed based on the exact organisms involved. IMO, Seachem is in tghe right ballpark for need #1 at 1 :0.05, but falls far short since it does not account for salinity changes (need #2).
1. Some magnesium is incorporated into calcium carbonate. I assumed an average ratio (by weight) of 1 for calcium and 0.065 for magnesium. Coralline can have a ratio as high as 1:0.12 . Some corals are far lower. This need for magnesium accounts for 1/3 of the magnesium I add to my recipe. Seachem uses a similar ratio to this part alone.
2. The second need for magnesium relates not to the incorporation into calcium carbonate, but the drop in magnesium when you readjust the salinity that is rising from the addition of sodium and chloride (and other counterions that are not consumed). This need for magnesium is BIGGER than the need for that incorporated. It is 2/3 of the total magnesium added to my recipe.
here's my discussion of the topic:
http://reefkeeping.com/issues/2006-02/rhf/index.php
After adding 594 grams of baking soda (1 gallon of Recipe #1), we will have added 163 grams of sodium. In natural seawater, magnesium is present at about 12.0% of the sodium concentration (by weight). In order to match the magnesium additions to the sodium additions to leave them in a natural ratio, we need to add 12% of 163 grams, or 19.5 grams, of magnesium for every gallon of the two-part additive that we add.
Additionally, we may want to account for magnesium that is actually incorporated into the coral skeletons. For this calculation, I have assumed that the amount of magnesium incorporated is about 6.5% of the calcium level (by weight), or about 2.5% of the skeleton by weight. In the course of adding this gallon of both parts of the two part supplement, we added 141 grams of calcium, so we need to add 0.065 x 141 = 9 grams of magnesium to account for this deposition.
The magnesium parts of the recipe are designed to add enough magnesium so that it is not depleted by either of the two means described above. Because the magnesium supplement (either version) is 47,000 mg/L in magnesium, we need to add (9 +19.5) grams/47 g/L = 610 ml of the magnesium solution for each gallon of the other parts of Recipe #1.
