Archived Relative Importance of Components in the …

Often aquarium keepers utilize RO water (which is excellent when used correctly), but often fail to add anything more than immediate electrolytes. It is helpful to also add a more complete and often continuous supply of mineral cations to this RO water. More importantly; ALL RO water should have KH Buffers added or problems of water stability WILL be encountered.
Worse, which should never be done, is the use of products that soften water by driving out mineral Cations or water from home water softeners which does the same. The result is water that CANNOT maintain essential mineral ions (even if added back) and a poor Redox balance!

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When magnesium levels remain sufficiently high, then the body can inhibit the enzyme HMG-CoA reductase whenever necessary. However, when there is magnesium deficiency, cholesterol synthesis increases because the conversion of HMG-CoA to mevalonate is enhanced (i.e., lacking inhibition of the enzyme HMG-CoA reductase by magnesium). Without the presence of sufficient magnesium, the body cannot prevent the synthesis of cholesterol from proceeding beyond amounts normally required for synthesizing important steroid hormones (estrogen, progesterone, testosterone, glucocorticoids, etc.) and Vitamin D. In these circumstances, cholesterol synthesis proceeds uninhibited by enzyme deactivation (i.e., at a greater rate than would be possible if adequate levels of magnesium were available to deactivate the enzyme HMG-CoA reductase and thereby block conversion of mevalonate into cholesterol). When magnesium levels are too low, your body loses the ability to inhibit cholesterol production, which boosts your synthesis of cholesterol. Maintaining optimum levels of magnesium has the opposite effect...your cholesterol production is regulated, controlled, and selectively inhibited, as needed!

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(The section following this section deals with Calcium specifically

General hardness (GH or dGH) refers to the dissolved concentration primarily of calcium, magnesium and other mineral ions.
Both Calcium and magnesium are important for proper internal osmotic processes in fish (and invertebrates).
More specifically, 1 dGH is defined as 10 milligrams (mg) of calcium oxide (CaO) per liter of water. Since CaO has a molar mass of 56.08 g/mol, 1 dGH is equivalent to 0.17832 mmol per litre of elemental calcium and/or magnesium ions.

A rock is an aggregate of one or more minerals or mineraloids

It may be seen that traditional cultures whose diets include an abundance of magnesium chloride do not appear to suffer from the high incidences of diseases of the heart that are witnessed in cultures or among peoples where modern western diets are predominant, or in places where the consumption of overprocessed, nutritionally depleted (and magnesium-depleted) food sources has become prevalent, that is, throughout much of the civilized world. Providing sufficient magnesium to the body is one of the best ways to achieve and maintain optimal cardiovascular health. Using magnesium chloride transdermally (i.e., across the skin) is the most convenient and effective way to boost magnesium levels, thereby making it available for improving the health of your heart!

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More importantly as to respiration, the surface epithelia of gills and body surfaces are protected from direct interaction with the environment by mucous and intercellular junctions.
Fish mucous has been postulated to have calcium binding properties.
Mucous is a glycoprotein and could serve as a calcium chelating agent retarding ion loss from epithelial cells as a charged surface coat or barrier and thus is dependent on calcium for normal function.
Intercellular junctions are specialized areas of attachment between epithelial cells preventing the loss of ions and fluids from the membrane which bathes and surrounds the cells beneath.

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Moreover, it is notable that the Japanese consume magnesium chloride in the form of nigari, a white powder that is created from seawater after the sodium chloride has been removed, and the seawater is evaporated. The Chinese consume a similar culinary item known as lushui. Nigari and lushui consist primarly of magnesium chloride and include some trace minerals. Nigari and lushui are added to drinking water, miso soups, steamed vegetables, etc. These ocean-derived mineral condiments are also used as coagulants in the production of tofu.