This is due to fact that a higher pH in sole (alkaline) can reduced the availability of micro-nutrients, such as Fe, Mn, Zn, Cu, B, etc, due to their inability to dissolve in the alkaline soil solution. Also a low soil pH (acidic) can result in nutrient toxicity as some of the micronutrients can become plant available in excessive amounts. Furthermore, low soil pH can also inhibit the soil microbes that convert N and P into plant available forms. For most plants, the ideal pH is somewhere between 6.0 and 7.0 with exceptions in both directions on the pH scale.
As to why aluminum and iron are more readily available nutrients for fish/plants compare to calcium and magnesium, is due to the fact that aluminum and iron are fairly reactive in acidic solution, thus they tend to form oxides, such as Iron (III) Oxide (Fe2O3), Al2O3, and so forth. This kind of reaction tend to product less of hydrogen gases, which would be see in reactions of alkali and alkali earth metals with acid. Thus, their environments are much conducive to fish and plants. Calcium and Magnesium are alkali earth metals, thus, they react vigorously with an acid solutions and produce more hydrogen gas in a solution. This higher concentration can be very toxic to marine enamels and plants.
