Redfield Ratio

Most ponds do not have any silica limitation since they are shallow and the pond bed is soil or there is input of water from rivers or wells.Iron gets oxidised in water and becomes unavailable to Diatoms. Some biotech product keeps the Iron and other trace metals stable in water. Another role of silica is to ensure that Diatoms not other phytoplankton consume the Iron and other trace metals.

Just to put this in perspective -Human beings require about 50 to 100 milli grams of Iron per day. Non availability of this results in millions of people (especially women) being anemic.1 atom of iron fixes 300000 atoms of CO2 during photosynthesis. http://en.wikipedia.org/wiki/Iron_fertilization "The Redfield ratio describes the relative atomic concentrations of critical nutrients in plankton biomass and is conventionally written "106 C: 16 N: 1 P."

This expresses the fact that one atom of phosphorus and 16 of nitrogen are required to "fix" 106 carbon atoms (or 106 molecules of CO2). Recent research has expanded this constant to "106 C: 16 N: 1 P: .001 Fe" signifying that in iron deficient conditions each atom of iron can fix 106,000 atoms of carbon,[25] or on a mass basis, each kilogram of iron can fix 83,000 kg of carbon dioxide."

1 kg of Iron can fix 83000 kgs of CO2, i.e., 22600 kgs of Carbon.
This gives about 45000 kgs of phytoplankton biomass.
This can give about 4500 kgs of fish biomass at 10% conversion rate.

The O2 released is about 60400 kgs (83000 - 22600), this increases the DO of water.

1 acre of water 1 meter deep is about 4 to 5 million liters of water.
If target DO is 6 mg / liter, the oxygen requirement is 30 kgs.
Iron requirement is 0.50 grams.

Oxygen is consumed by bacteria, zooplankton, fish, etc., the Iron required to replace this is a less than a gram per day per acre.

Diatoms are responsible for 25% of all oxygen in the atmosphere, this is because the O2 produced is so much that it escapes from the oceans into atmosphere.

Correspondingly Carbon in oceans is 38000 billion tons compared to just 700 billion tons in atmosphere.
Oxygen makes up 21% of Atmosphere but CO2 is just 0.04% (390 ppm).