PhosGuard 4 L — Spherical Anti-Phosphate Aluminum Oxide | Seachem

Seachem PhosGuard 4L is the largest size in the PhosGuard range, 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 control market—prevents clumping and maximizes water contact. The 4L size is the most economical choice for large reef systems, multiple setups, or aquarists who maintain an ongoing phosphate control program.

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
• 4L size — the most economical in the range, ideal for large systems and long-term use
• Inert and safe — can remain in the filter indefinitely without risk

Use in reef and marine aquariums

High phosphate levels promote the growth of unwanted algae and hinder 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 efficiency, not low capacity. Seachem recommends using small amounts continuously rather than large amounts intermittently. The 4L format provides enough media for numerous treatment cycles over several months. PhosGuard performs best in saltwater and alkaline pH. Under reef conditions (pH close to 8), no detectable soluble aluminum is released.

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 (risk of excessive heat).
• 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.
• PhosGuard can be removed, dried, and put back into service as long as it is not exhausted.

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
• Not recommended for phosphate-buffered freshwater aquariums

Comparison of Seachem anti-phosphates

• PhosGuard (aluminum oxide): 0.87 mg PO4/g — rapid absorption, spherical shape
• PhosBond (GFO + aluminum oxide hybrid): 1.19 mg PO4/g — intermediate capacity
• PhosNet (GFO): 1.41 mg PO4/g — highest capacity per gram

Available formats

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

Why choose PhosGuard from ReefSolution?

The 4L format offers the best cost per liter in the PhosGuard range for serious aquarists. Place it in the Seachem The Bag for optimal use. For higher binding capacity per gram, consider Seachem PhosBond or Seachem PhosNet. Monitor your phosphate levels with the Hanna Checker Phosphate or the Salifert Phosphate Test Kit.

Frequently Asked Questions

How many treatments does the 4L format offer?

At the standard dosage of 85 ml per 200 L of seawater, the 4 L format provides approximately 47 treatment doses. For a 200 L aquarium with controlled phosphates, this represents more than a year of continuous use with replacement as needed based on testing.

Is the 4L format the most economical in the range?

Yes. The 4L offers the best cost per liter of media, ideal for large reef systems, multiple setups, or aquarists who maintain a long-term, continuous phosphate control program.

How to store PhosGuard 4 L between uses?

Unused PhosGuard can be stored dry in its original container. Used but not exhausted media can be removed, dried, and reused. It does not release phosphates or silicates and is inert and safe.