I just read an article that I thought was pretty interesting and know of one person it has helped so far. Specially this quote below:
A nutrient antagonism is when an excessive concentration of one nutrient inhibits the uptake of another. Since K, Ca and Mg have similar properties, and are taken up in a similar fashion, too much of one nutrient can inhibit the uptake of another nutrient. For example, if the concentration of Ca gets too high, it can impede the uptake of Mg. Or if the K concentration gets too high, Ca uptake can be inhibited. The ideal ratio of K:Ca:Mg to each other in hydroponic nutrient solutions to avoid uptake varies a bit, but is usually three to five parts K and Ca to one part Mg (3-5K:3-5Ca:1Mg). However, this tends to vary with plants. For example, lettuce and leafy greens do well when Ca and K are balanced with each other. Fruiting crops do well with a greater proportion of K.
The whole article can be found here;
https://www.producegrower.com/article/hydroponic-production-primer-potassium-calcium-magnesium/
So with that in mind, why do people recommend to use calmag with maxibloom? The bloom already has the appropriate ration of K, ca and mg. My friend was getting a calcium deficiency from using calmag with maxibloom because the calcium was being blocked out from excessive mag... He dropped to just bloom and no more problems. He was using ro water btw...