API Marine Algaefix 473ml

• Controls nuisance marine algae – targeted at hair, string and fuzz algae fouling glass, rock and décor.
• Labelled safe with live corals and invertebrates – API states it will not harm snails, clams, scallops, shrimp, anemones, sea cucumbers, feather dusters, coralline algae, soft corals or hard corals.
• Simple dosing – 1 mL per 10 US gallons (37.9 L), repeated every three days until the algae is controlled, then once a week to maintain it.
• Works out to under 1c per litre of tank water treated, based on the manufacturer's stated coverage and the current price.
• 473 mL (16 fl oz) bottle – API states this treats up to 4,800 US gallons (≈18,170 L) as a single dose; a tank dosed repeatedly every three days will use the bottle faster than that figure implies.

Reproduced from API's label directions:

• Initial treatment: 1 mL per 10 US gallons (37.9 L), or 5 mL per 50 US gallons (189 L), added directly to the aquarium.
• Repeat this dose every three days until the algae is under control.
• Once controlled, switch to one maintenance dose per week at the same rate.

API Marine AlgaeFix treats the algae that's already growing – it doesn't remove the phosphate or nitrate feeding it. Pairing it with regular water changes and nutrient export (a protein skimmer, GFO, or a nitrate/phosphate reducer) stops the algae coming straight back once dosing stops.

Suitable for:

• Marine fish-only and reef aquariums with nuisance hair, string or fuzz algae
• Systems that hold live corals, snails, clams, shrimp, anemones and other invertebrates, dosed at the labelled rate

Not suitable for:

• Freshwater aquariums – use API Freshwater AlgaeFix instead, which is a different product with its own label
• Replacing nutrient control – it treats the algae, not the phosphate or nitrate feeding it

API's product listing names the active ingredient as Poly[oxyethylene(dimethyliminio)ethylene(dimethyliminio)ethylene dichloride], a polymer algaecide. A published EPA pesticide label for API's Algae Fix product line states this active at 4.50%, but that label could not be confirmed as specific to the Marine AlgaeFix formulation sold here rather than a related freshwater or pond product in the same range, so the percentage is not stated as fact on this page. Checked 7 August 2026.

• Keep out of reach of children.
• Ensure strong aeration while dosing – algae die-off can draw down oxygen.
• Do not exceed the maintenance dose without confirming your tank's response first.

• Red Sea Marine Care Multi Test Kit – check phosphate and nitrate to find what's actually feeding the algae.
• Seachem Prime – for conditioning the water changes that support nutrient export alongside dosing.

This is a consumable, so no manufacturer warranty period applies. That does not affect your rights under the Australian Consumer Law: if the product is faulty, unsafe or not as described, you are entitled to a remedy.

Ships as standard freight.

Is API Marine AlgaeFix safe for my reef tank?

API labels it safe for use in marine aquariums containing live corals, invertebrates and fish, stating it will not harm snails, clams, scallops, shrimp, anemones, sea cucumbers, feather dusters, coralline algae, soft corals or hard corals when dosed as directed.

How often do I dose it?

Every three days until the algae is under control, then once a week as a maintenance dose, at 1 mL per 10 US gallons (37.9 L) each time.

Will the algae come back once I stop dosing?

It can, if the phosphate or nitrate feeding it hasn't been addressed. AlgaeFix treats the algae itself; pairing it with water changes and nutrient export gives a longer-lasting result.

Can I use the freshwater AlgaeFix in my marine tank instead?

No. API sells separate freshwater and marine formulations with their own labels and dose rates – use the one labelled for your water type.

How long will one bottle last?

API states the 473 mL bottle treats up to 4,800 US gallons (≈18,170 L) as a single dose. Because the label calls for repeat dosing every three days during initial treatment, an actively-treated tank will use the bottle faster than that headline figure suggests.