Phosphates stuck at 0.2 in 5-month-old tank

[Miami Reef]

Rocks (calcium carbonate) don’t absorb phosphates; they adsorb phosphates, similarly to the mechanism of how GFO reduces phosphates in aquariums.

 

[DanyL]

The reason to be precise with this in our case, is that people who thing that rock and sand can just let go of po4 for some unknown reason at any time are getting it wrong. This does not happen. If they think this, then they can think that the rock/sand is why their po4 is going up, which is not the case... so if they know, they can find the real reason, which is likely lack of export and possibly overfeeding.

This is exactly the explanation I was looking for.

I knew very well how PO4 binding/unbinding behaves, so much so that I guess I just took it for granted.

Thanks for taking your time to write this explanation.

 

[JHSteepat]

Here's some fun reading.

Millero, FJ, et al. Adsorption and Desorption of Phosphate on Calcite and Aragonite in Seawater; March 2001; Aquatic Geochemistry 7(1):33-56​

What I take from this (and what was my understanding and seems to be the consensus above) is that if you remove the phosphate from the water, it desorbs fairly quickly from the aragonite, but reaches an equilibrium if you keep adding phosphate to the system and maintain phosphate levels (lack of export, feeding, etc.). I believe that because you have water contacting the rock, the best you can do is keep the phosphate in the water low to drive the equilibrium towards release of phosphate from the aragonite. The lower you can keep the in-water phosphate, the more you can desorb from the rock, whether you use lanthanum, GFO, ATS, chaeto, etc.

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[jda]

While you still have a good amount of po4 in the water, the rock will unbind to equilibrium again usually in under 24 hours. This is mostly surface and easy-to-get-to parts of the aragonite structure. As you lower levels of tank water po4, then it can take a week or more for the aragonite to unbind as the water has to enter deeper areas.

For example, if you have .25 in your water column and you did a 100% water change and tested at .00 or .01 instantly after the water change, in about 24 hours, you would be at .24 in your water column again. You removed some po4 from the water column, but the rock has MUCH more bound.

The rock can usually bind po4 as the water column rises what seems like almost instantly, or at least very quickly.