What's a good target phosphate for a reef tank?

0.03–0.08 ppm PO₄ (or 0.01–0.025 ppm phosphorus). Below 0.03 ppm risks dinoflagellate blooms; above 0.08 ppm risks nuisance algae and SPS browning. Stable parameters matter more than absolute values — corals tolerate 0.05 ppm stable better than 0.02 ppm fluctuating.

Which is better — GFO or PhosGuard?

GFO for most reefs because it's iron-based, runs cleaner in a reactor, and has more flexible flow tolerance. PhosGuard for nano tanks without reactors (passive bag in sump) or for keepers who specifically want to avoid iron contributions from media.

Yes — crashing PO₄ below 0.03 ppm risks dinoflagellate outbreaks. Start GFO at half the calculated charge, run it for 2 weeks, test, then add more if needed. Never load the full charge on day one if PO₄ is above 0.5 ppm.

Why does lanthanum require a filter sock?

Because it precipitates phosphate as a fine white cloud that's bigger than free PO₄ ions but smaller than typical filter floss. A 1-µm sock or polishing pad catches the precipitate before it settles on coral or blows through the skimmer. Without the sock, you're moving phosphate around the tank, not removing it.

How often should I re-test PO₄ after dosing?

GFO/PhosGuard: weekly. Lanthanum: 48 hours after the dose, then weekly. Use a low-range test (Hanna ULR Phosphate, Salifert) — drop-test kits like API are too imprecise at reef-target levels.

Calculator

Reef Phosphate Remover Dose Calculator (GFO / PhosGuard / Lanthanum)

Name: Reef Phosphate Remover Dose Calculator (GFO / PhosGuard / Lanthanum)
Author: reefcalcs

Three different phosphate-binding media solve the same problem at different cost and risk profiles. GFO (granular ferric oxide) is the reef default; Seachem PhosGuard is the older aluminum-based alternative; lanthanum chloride is a single-shot precipitant that drops PO₄ fast but punishes overdoses. This calculator gives the correct dose for all three on one page, with per-media replacement schedules and the warnings each one needs.

JavaScript is disabled in your browser, so the interactive calculator below won't run. The math, formula, and worked example are all written out further down this page under “How this is calculated.” Enable JS (or open this page in another browser) to use the live calculator.

Whether you compute by hand or in the calculator, the same caveat applies: verify against your test kit and the linked primary source before dosing. reefcalcs is reference information, not veterinary or engineering advice.

Phosphate-remover dose

Tank volume (US gal)

Phosphate media

Current PO₄ (ppm)

From a Hanna ULR or Salifert reading.

Target PO₄ (ppm)

Reef target: 0.03–0.08 ppm. Below 0.03 risks dino blooms.

Initial charge

375mL

Replace every

6weeks

When PO₄ stops dropping, swap the media.

• Tumble in a reactor at 1–2 grains-per-second flow. Slow flow at first — fast tumble shaves GFO into the water column.
• Watch for rust-coloured fines in the display the first 24 hours. If excessive, increase pre-rinse next time.

Standard GFO at 250 mL per 50 gallons (BRS dosing chart); BRS High-Capacity at half that volume. PhosGuard at 85 g per 100 L (Seachem label). Lanthanum chloride at ~2 mL of 5 % LaCl₃ per ppm PO₄ per gallon (SeaChem PhosControl + Randy Holmes-Farley lanthanum chemistry).

How this is calculated

Standard GFO (BRS Regular, Phosban):
  mL_initial = tank_gal × 5
  Replace every 6 weeks.

BRS High-Capacity GFO (~2× absorption per gram):
  mL_initial = tank_gal × 2.5
  Replace every 6 weeks.

Seachem PhosGuard:
  grams_initial = tank_gal × 3.22
  (Seachem label: 85 g per 100 L)
  Replace every 4 weeks.

Lanthanum chloride 5 % (single shot):
  mL = tank_gal × (current_PO4 − target_PO4) × 2.0

  CAP: single dose < 0.5 ppm PO₄ reduction.
  Drip ≤ 1 mL/min into high-flow zone.
  Polish return with 1-µm sock/pad for 48 h.

GFO works by ion exchange — iron in the oxide swaps places with phosphate in the water, binding it to the granules. Flow rate matters: too slow and the column channels (water finds paths through old wet granules and bypasses fresh ones); too fast and the granules grind against each other producing rust-coloured fines. The BRS rule of "1–2 grains-per-second tumble" gives the right window.

PhosGuard is functionally similar but aluminum-based. It works in low-flow passive bags better than GFO, which is useful in nano sumps without dedicated reactors. The historical concern about aluminum leaching has been mostly debunked in modern literature, but some clam-keepers and iodine-dosers still avoid it on principle.

Lanthanum chloride is mechanically different — it precipitates phosphate as insoluble lanthanum phosphate that you then filter out mechanically. It's the fastest method (visible PO₄ drop in hours, not weeks) and the most dangerous if overdosed. The standard reef-safe dose is ~2 mL of 5 % LaCl₃ per ppm PO₄ per gallon. SeaChem PhosControl uses a different concentration; read the bottle label.

FAQ

What's a good target phosphate for a reef tank?: 0.03–0.08 ppm PO₄ (or 0.01–0.025 ppm phosphorus). Below 0.03 ppm risks dinoflagellate blooms; above 0.08 ppm risks nuisance algae and SPS browning. Stable parameters matter more than absolute values — corals tolerate 0.05 ppm stable better than 0.02 ppm fluctuating.
Which is better — GFO or PhosGuard?: GFO for most reefs because it's iron-based, runs cleaner in a reactor, and has more flexible flow tolerance. PhosGuard for nano tanks without reactors (passive bag in sump) or for keepers who specifically want to avoid iron contributions from media.
Can I overdose GFO?: Yes — crashing PO₄ below 0.03 ppm risks dinoflagellate outbreaks. Start GFO at half the calculated charge, run it for 2 weeks, test, then add more if needed. Never load the full charge on day one if PO₄ is above 0.5 ppm.
Why does lanthanum require a filter sock?: Because it precipitates phosphate as a fine white cloud that's bigger than free PO₄ ions but smaller than typical filter floss. A 1-µm sock or polishing pad catches the precipitate before it settles on coral or blows through the skimmer. Without the sock, you're moving phosphate around the tank, not removing it.
How often should I re-test PO₄ after dosing?: GFO/PhosGuard: weekly. Lanthanum: 48 hours after the dose, then weekly. Use a low-range test (Hanna ULR Phosphate, Salifert) — drop-test kits like API are too imprecise at reef-target levels.

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Sources & references

01
Bulk Reef Supply — GFO product info and dosing chart
https://www.bulkreefsupply.com/reef-calculator/gfo-calculator
02
Randy Holmes-Farley — Phosphate and the reef aquarium
https://reefkeeping.com/issues/2002-09/rhf/feature/index.php
03
Seachem — PhosGuard product page (dose: 85 g per 100 L)
https://www.seachem.com/phosguard.php
04
Seachem — PhosControl (lanthanum) usage chart
https://www.seachem.com/phoscontrol.php
05
Randy Holmes-Farley — Chemistry and the aquarium: Lanthanum
https://reefs.com/magazine/chemistry-and-the-aquarium-fluoride-aluminum-and-lanthanum/

Last reviewed 2026-05-25