PhosGuard 250 ml — Anti-Phosphate Spherical Aluminum Oxide | Seachem

Seachem PhosGuard 250 ml is a spherical aluminum oxide-based filter media that quickly removes phosphates and silicates from marine and freshwater aquariums. Its unique spherical bead shape—unique in the phosphate remover market—promotes optimal water flow without compaction, increasing efficiency and capacity compared to competing granular media. The 250 ml bulk format treats up to 600 L (150 US gallons) of saltwater and offers the flexibility to dose as needed.

Main advantages

• Rapidly removes phosphates and silicates — capacity of 0.87 mg PO4/g
• Unique spherical shape on the market — optimal water flow, no compaction
• Irreversible bond — does not release phosphates or silicates into the water
• 250ml bulk format — divisible into portions according to aquarium size
• Inert and safe — can remain in the filter without risk

Use in reef and marine aquariums

High phosphate levels promote the growth of unwanted algae and impair coral growth and coloration. PhosGuard absorbs phosphates very quickly—reaching full capacity in 4 to 5 days in the presence of high phosphates. This rapid replacement rate is a sign of effectiveness, not low capacity. Seachem recommends using small amounts continuously rather than large amounts intermittently. PhosGuard works best in saltwater and at alkaline or neutral pH. Under reef conditions (pH close to 8), no detectable soluble aluminum is released, according to Seachem's controlled studies.

Instructions for use

• Pre-wet by adding the product to twice its volume of fresh water, then rinse. Do not add water to the dry product. Do not use seawater for the initial wetting.
• Place in a filter bag (recommended) in a cartridge filter, media chamber, box filter, or any high-flow area — do not use in a media reactor
• Dosage: 85 ml (⅓ cup) per 200 L (50 US gallons) of seawater or 400 L (100 US gallons) of freshwater
• Leave in place for 4 days, then test the phosphates. If the concentration has not decreased to 0.02 mg/L, replace the PhosGuard.
• If the levels remain under control, the product is not depleted — leave it in place and test weekly.

Manufacturer's precautions

• May generate heat upon first contact with water — always pre-wet with fresh water when adding the product to the water, never the other way around
• The yellowing of the medium indicates the absorption of organic compounds, not the depletion of phosphates.
• Do not overdose — too rapid a drop in phosphates can stress sensitive corals
• Aluminum oxide is NOT regenerable — cooking does not restore its absorption capacity

Comparison of Seachem anti-phosphates

• PhosGuard (aluminum oxide): 0.87 mg PO4/g
• PhosBond (hybrid GFO + aluminum oxide): 1.19 mg PO4/g
• PhosNet (GFO): 1.41 mg PO4/g

Available formats

• 100 ml (sachet)
• 250 ml
• 500 ml
• 1 L
• 2 L
• 4 L

Why choose PhosGuard from ReefSolution?

The 250 ml size is the versatile format for medium-sized reef aquariums. Place it in the Seachem The Bag for optimal use. For superior binding capacity, consider Seachem PhosBond or Seachem PhosNet. Monitor your phosphate levels with the Hanna Checker Phosphate or the Salifert Phosphate Test Kit.

Frequently Asked Questions

Is the PhosGuard 250ml loose or pre-packaged?

The 250ml size is sold loose. A filter bag is recommended, such as the Seachem The Bag. This allows you to divide the quantity into portions suitable for the volume of your aquarium.

Does PhosGuard release aluminum that is toxic to corals?

No. Seachem's controlled studies demonstrate that under reef conditions (pH close to 8), no detectable soluble aluminum is released. Even at three times the prescribed dosage, soft corals remain unaffected.

Does PhosGuard also absorb silicates?

Yes. When phosphate levels are low, PhosGuard absorbs silicates from the system. High silicate levels can promote the growth of brown diatoms, which are common in new aquariums.