Absorbs phosphate.
Absorbs silicates.
Does not absorb ammonia/ammonium.
Does not absorb nitrate.
Does not absorb nitrite.
Does not absorb CO2.
Does not produce oxygen.
Does not produce pods.
Does not produce amino acids, vitamin C, or carbs.
Is a non-living chemical (iron).
Is expensive, and must continue to be replaced.
Needs some place to go (bag, reactor, etc).
Is dark brown.
If spilled into water is very hard to remove.
Animals cannot eat it.
Can lower phosphate too low, resulting in coral bleaching.
Have been reports of GFO causing HLLE disase in fish.
Capacity is reduce from first day, and continues dropping.
Does not provide any useful end product.
Heavy iron particles can clog your drain.
The iron can fuel nuisance algae growth.
Does not need light/electricity.

Algae Scrubbers:

Absorbs phosphate.
Does not absorbs silicates.
Absorbs ammonia/ammonium.
Absorbs nitrate.
Absorbs nitrite.
Absorbs CO2.
Produces oxygen.
Produces pods.
Produces amino acids, vitamin C, and carbs.
Is living, and growing.
Is free, and replaces itself.
Needs some place to go.
Is green or brown.
If spilled into water is easy to remove.
Animals eat it.
Cannot lower phosphate too low.
No reports of causing HLLE disase in fish.
Capacity increases from first day, as growth develops.
Provides useful end product (algae).
Very light particles go down drain easily, and dissolve.
Consumes iron.
Needs light/electricity (pennies per month).