Seachem Flourish Phosphorus

• Targeted phosphorus, no nitrate – corrects a phosphate deficiency without adding nitrate, so the two nutrients can be dosed independently.
• Dose to your test kit, not a guess – Seachem recommends adjusting the dose to a phosphate test result rather than dosing blind, because dosing without testing is the main cause of pest algae with this product.
• Precision dosing formula included – for aquarists who want an exact phosphate increase rather than the standard maintenance dose.
• Concentrated – every size treats water for under 1c per litre, and the cost per litre drops further as pack size goes up.
• Reported safe for shrimp – Seachem states it is safe for invertebrates such as shrimp when dosed as directed.

Active constituent: potassium phosphate solution, supplying available phosphate (P₂O₅) 0.3% and soluble potash (K₂O) 0.2%.

All five sizes cost well under 1c per litre of water treated, and the cost per litre falls as the bottle gets bigger – the 4 L pack works out cheapest per litre treated, the 100 mL bottle the most expensive. Buy small if you're testing whether your tank needs regular phosphorus dosing at all; buy the 2 L or 4 L if you're already dosing weekly across a large or heavily planted system.

Standard maintenance dose: 2.5 mL (half a cap) per 80 L, once or twice weekly, or as needed when plants show deficiency signs such as stunted growth or dark-green colouring. Seachem's own guidance is to dose against a phosphate test kit reading rather than a fixed schedule, targeting a phosphate level of roughly 0.15–1.0 mg/L.

For a precise increase, Seachem publishes a dosing formula: 0.8 × tank volume in US gallons × desired phosphate increase in mg/L = mL to dose.

• Suitable for freshwater planted aquariums, including high-tech CO2-injected setups and low-tech tanks.
• Reported safe for freshwater invertebrates such as shrimp when dosed as directed.
• Not intended as a stand-alone fertiliser – most planted tanks also need a nitrogen, potassium and trace-element source; Seachem does not publish reef or marine dosing guidance for this product, so treat it as a freshwater planted-tank product only.

A minor overdose is typically not harmful, but a large one can shift nutrient levels quickly enough to stress fish, plants and beneficial bacteria – measure doses rather than pouring by eye, and keep dosing tied to test results rather than a guess.

• Seachem Flourish Potassium – corrects potassium deficiency, the nutrient most often short alongside phosphorus.
• Seachem Flourish Trace – covers the trace elements this product doesn't supply.
• A phosphate test kit – needed to dose this product against a reading rather than a fixed schedule.

Covered by your rights under Australian Consumer Law in addition to any manufacturer guarantee. Ships Australia-wide from North Lakes, QLD.

Will this cause algae?

Only if it's dosed without testing. Phosphate is a plant nutrient, but oversupplying it relative to what your plants can use is a common cause of pest algae – dose against a test kit reading rather than on a fixed schedule alone.

Does it add nitrate?

No. It's formulated to supply phosphorus without nitrate, so it can be dosed independently of your nitrogen fertiliser.

Is it safe for shrimp?

Seachem states it is safe for invertebrates such as shrimp when dosed as directed.

What size should I buy first?

Start with the 100 mL or 250 mL bottle if you haven't dosed phosphorus before – it lasts a long time at the standard dose, and lets you confirm your tank needs it before committing to a larger pack.

How is the dose measured?

By volume against your tank size – 2.5 mL per 80 L as a maintenance dose, or Seachem's formula (0.8 × US gallons × desired mg/L increase) for a precise target.