DIY Phosphate Dosing Recipe

[Miami Reef]

Sometimes phosphate can be so stubbornly low that the only way to meaningfully raise it is by directly dosing phosphate.

When you feed foods, you’re adding nitrogen, carbon, and loads of organics. This effect is compounded if you’re trying to offset rocks that initially work against you by binding up most of the PO₄ until a higher equilibrium is met. It’s much cleaner and more cost-effective to purchase pure sodium phosphate online than to use hobby-grade products with unknown purities and byproducts.


You can mix sodium phosphate with nitrate, ammonium, or even add it to an ATO; however, phosphate should not be mixed with limewater (kalkwasser), as it will completely precipitate as calcium phosphate. You may need to dose more than expected, as calcareous sand and rock can bind much of it, especially when phosphate levels are near zero.


Sodium Phosphate Dibasic (Na₂HPO₄) Recipe:

Sodium phosphate dibasic (Na₂HPO₄) is 66.9% phosphate (PO₄³⁻) by weight.

0.02 ppm in 100 L = 2 mg of PO₄³⁻.
2 ÷ 0.669 = 2.99 mg of Na₂HPO₄.

Dissolve 3 g of Sodium Phosphate Dibasic (Na₂HPO₄) in 1 L of freshwater.

Dose 1 mL per 100 L to raise PO₄ by 0.02 ppm.

Note: Monobasic, dibasic, and tribasic forms are interchangeable. Concentrations vary slightly but are functionally similar when accounting for rock adsorption.

If calculators are easier, you can follow this link. Select “potassium phosphate.”

James' Planted Tank - Dosing Calculator

 

[Miami Reef]

Post is now live. :)

 

[Miami Reef]

If phosphate is reading 0.00 ppm, corals are stressed, and dinoflagellates are taking hold, I recommend immediately adding 0.10–0.15 ppm PO₄.


Rocks will bind much of the phosphate. Adding tiny amounts and testing daily, only to watch levels crash back to zero, wastes time, test reagents, and prolongs coral starvation.


It’s more efficient to bolus dose upfront to speed up rock saturation.

 

[rishma]

I support the 0.1 to 0.15 daily dose when readings are very low. Excellent advice.

 

[ReefQuestCorals]

Good write up!

Saved it for later :) Will use this method to raise phosphates in my new system

 

[Miami Reef]

Thanks guys!

 

[Featherweight]

What is the recipe for trisodium phosphate?

 

[Miami Reef]

What is the recipe for trisodium phosphate?

The same recipe as above. :)

They are nearly identical.

 

[Featherweight]

Excellent. I assumed as much but figured we'd get it on record too!

 

[Featherweight]

This is working well for me, thank you

 

[Miami Reef]

This is working well for me, thank you

I’m glad it’s working.

 

[Gumbies R Us]

Thank you for sharing your knowledge with us!

 

[Miami Reef]

Thank you for sharing your knowledge with us!

Of course! :)

 

[Ken S]

Thanks for this post. I am having trouble keeping phosphate & nitrate at acceptable levels. I am using Brightwell products to dose. It's happening just as you explained, test daily, add recommended dose on bottles, repeat next day. If I skip a dose, everything drops by half. Never have I seen phosphate above .11 & it usually hovers around half that. Same with nitrate, highest reading ever seen is 14.0 but usually hovers around half of that or less. I've upped amount & frequency of feedings, cut back refugium light to on 8hrs/day and not changing sock nearly as often.

Can i safely double the dose of these Brightwell products to spike levels up a it? Or am I asking for trouble doing that?
Thank you!

 

[Bohemian Waxwing]

3g into 1L! That’s awesome. A small container will last.

Any potential bad interactions with things like AFR, Kalk, Calcium nitrate etc?


Also, in terms of grades, is reagent grade considered a good candidate? I was looking at this: https://bioshopcanada.com/Products/Details/SPD307

 

[Miami Reef]

Can i safely double the dose of these Brightwell products to spike levels up a it? Or am I asking for trouble doing that?
Thank you!

Yes. You can definitely ramp up those dosages.

 

[Miami Reef]

Also, in terms of grades, is reagent grade considered a good candidate? I was looking at this: https://bioshopcanada.com/Products/Details/SPD307

Yes. That product is 99% pure which is good enough for phosphate dosing.

 

[Jasonak]

I dont have a scale ,,,,any idea what that would be in tea or table spoons ?
@Miami Reef

 

[Randy Holmes-Farley]

A teaspoon being 5 grams is a reasonable guess for many materials. :)

 

[Miami Reef]

Someone was wondering what the difference would be if you used trisodium instead of dibasic with the recipe above.

Using trisodium phosphate instead of dibasic will raise your PO₄ by about 0.017 ppm per 0.02 ppm dose. The difference is minor and the two are effectively interchangeable.

Trisodium phosphate delivers 86.6% as much phosphate per gram as dibasic.

Just a scale to show how minor the difference between them is.

This is the trisodium chemical. Like the dibasic, it is also anhydrous.