Cobalt
Cobalt is found in natural seawater at parts per billion concentrations. Under the conditions of this procedure, cobalt (likely only free cobalt) is actively removed by GFO. See Figure 2.

Figure 2. Cobalt concentrations fell after GFO was introduced.
Copper (Free)
Copper in an aquarium is a double-edged sword. It is an essential trace element but is toxic at higher concentrations. Free and total copper were determined. See Figures 3 and 4.

Figure 3. Free copper removal by GFO. The Detection Limit of the test is 0.02 mg/L as Cu. The final reading (0.09 mg/L) is still above that usually seen in natural seawater.
Copper (Total)
Total copper is the sum of free and complexed copper.

Figure 4. Total copper fell due to removal of free copper by GFO.
Copper (Chelated or Complexed)
Copper, complexed with organic substances, can be determined by subtracting free copper from total copper. Figure 5 shows complexed copper is weakly removed, if at all, by GFO.

Figure 5. Chelated (complexed) copper is very weakly removed by GFO, if at all.
Iron
Since iron is a major component of GFO, and this substance is subject to weak grinding action within a fluidized reactor, it might be expected that the iron content of the aquarium water would increase, and indeed it did. Further analyses found the iron to be almost entirely in the ferric form, but perhaps more importantly, as mostly in the form of suspended particulates and not dissolved. See Figures 6 through 9.

Figure 6. Experiment #1 showed total iron initially rose and then fell.

Figure 7. Results of experiment #2 found total iron concentrations rose and fell very much like that seen in experiment #1.

Figure 8. Total iron minus ferrous iron equals ferric iron.

Figure 9. Very little of the total iron was soluble but instead found in the suspended form.
Manganese
Manganese is an essential element for plants and animals. Free manganese is actively removed by GFO, although the concentration of manganese in the spiked sample plateaued at ~0.07 mg/L at about 120 hours and thereafter. See Figure 10.

Figure 10. Manganese was actively removed by GFO. The analytical procedure can report Mn concentrations of 0.006-0.7 mg/L, while natural seawater contains ~0.001 to 0.01 mg/L.
Silica
GFO is advertised to effectively remove silica, and testing confirms this. Silica is quickly and effectively reduced in concentration, as its concentration fell to below the detection limit of the testing device (in essence 'zero'). See Figure 11.

Figure 11. Silica concentrations fell to below the detection limit of the instrument (essentially zero) inn less than 24 hours under the conditions of this experiment.
Zinc
Zinc (free and not complexed) is rapidly removed by GFO. At 104 hours, the concentration had fallen from ~6mg/L to 0.38 mg/L (a removal of ~94%). See Figure 12.

Figure 12. The Detection Limit of the analytical device is 0.02 mg/L as Zn.
pH
pH (the intensity of the basic or acidic natural of a substance) is known to be influenced through use of GFO. Figure 13 demonstrates this effect.

Figure 13. Use of GFO can cause rapid shifts in pH.
Phosphorus
As with many elements, phosphorus is essential for life but can cause problems - specifically that of enhanced algal growths - at relatively low concentrations. Hobbyists should be concerned with reactive or ortho-, phosphate as this is the form that fuels algae growth (this is the form most all 'test kits' report). On the other hand, total phosphorus is that bound with other substances and requires a specialized digestion process including heat and an acidic environment. See Figures 14 and 15.