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SILICA ADDITIVE QUALITY - Si and Value for Your Money
The chemistry of silica in hydroponic solutions is extremely complex but, to put things simply, plants can only uptake monosilicic/orthosilicic acid (H4SiO4). 
What this means is that only a percentage of any silica additive, when ¡t is applied to solution, is available for plant uptake; i.e. when silica is added to aqueous environments (e.g. hydroponic nutrient solutions) chemical processes/reactions occur whereby non-bioavailable forms of Si become bioavailable as H4SiO4 and vice versa.
This peaks in acidic
solutions (e.g. hydroponics where pH is below 7), no matter how much Si is added to solution, at no higher than 100-120 ppm dependent on prevailing conditions (higher concentrations of soluble monomers are only obtained above pH 11-12.). As I said, this is extremely complex chemistry; therefore, for reasons of expediency and to avoid being mugged by chemistry let’s not go there.
Suffice to say, silica in solution is inherently unstable and a high degree of it is non-bioavailable for plant uptake. Additionally, because silica ¡s inherently unstable it has the tendency to undergo changes ¡n solution which results ¡n a large degree of precipitation and/or insolubility over even short periods of time.
In recent times, certain suppliers of silica products have latched onto the inherently unstable nature of silica in hydroponic solutions and released what they claim to be stabilized silica additives, which they also claim means your plants receive more bioavailable mono/orthosilicic acid.