Calcium and magnesium deficiencies are among the most common problems in coco and in soft water, because coco holds onto calcium and because reverse osmosis and rainwater start with almost none. Silica is a separate job: it stiffens cell walls, which shows up as sturdier stems and better heat tolerance.
We stock cal-mag and silica from Green Planet, Flairform, Terra Aquatica, Cyco and Bloom, generally between $13 and $96 with most around $41.
Using them. Cal-mag is usually needed in coco and with RO or rainwater, and often unnecessary in hard tap water or a soil already limed, so test before you add. Add silica to the reservoir first and on its own, before anything else, and stir it through: silica products are alkaline and will react with concentrated nutrients if they meet undiluted, which is what causes the white sludge people blame on the bottle. Recheck pH after adding silica, because it will push it up.