Nitrates and Phosphates - Water Change %

[disaster999]

FWIW, water changes are not the best way to deal with phosphate. Even a 100% change may end up only lowering the phosphate level slightly.

Why is that? Sure I get that rocks and other things in the tank can leach phosphate back, but I cant wrap my head around the statement that a 100% water change will only lower the level slightly

 

[BryanM]

Why is that? Sure I get that rocks and other things in the tank can leach phosphate back, but I cant wrap my head around the statement that a 100% water change will only lower the level slightly

Apparently “rock” can hold quite a lot of phosphate.

I can only guess that it lets out X amount over time, but still has plenty to give.

This is why it takes time to reduce, and even after a large water change people often end up with the same levels

 

[Dan_P]

Hi guys,

I have a 350L reef tank and am super high on nitrates and phosphates.

Here are my parameters:
Nitrates - 75+
Phosphates - 0.9
Alk - 15.2
Calcium - 520
Magnesium - 1230
Salinity - 1.026
pH - 8.3

I really need to reduce these numbers because I have a lot of algae and some of my coral aren’t so happy.

What % water change should I do and how frequently to avoid impacting my fish and coral negatively?

I’ve changed 15% each day for the last two days.

Should I keep doing 15% daily or what?

Any help would be appreciated!

Just finished reading the thread and want to repeat that if you are interested in lowering the nitrate concentration, you will need to know the actual concentration. Carbon dosing and water change will show no effect if the measurement cannot read above 75+. How do you know that the value hasn’t dropped from 100 to 80 ppm with your efforts? Also, it is unlikely that you are generating much nitrate everyday. More likely your system is just very good at accumulating it, meaning, the removal rate is slower than the generation rate.

By the way, the aquarium is a beauty.

 

[Randy Holmes-Farley]

Apparently “rock” can hold quite a lot of phosphate.

I can only guess that it lets out X amount over time, but still has plenty to give.

This is why it takes time to reduce, and even after a large water change people often end up with the same levels

Yes. Bare surfaces of calcium carbonate (rocks, sand, etc.) will bind phosphate, and the binding amount relates to the concentration of phosphate in the water (and the exposed surface area). Less binds at lower concentrations

If phosphate is 0.2 ppm in the water, there may be many times that amount bound to the rock surfaces.

If you do a 100% water change, the concentration in the water is suddenly 0.00 ppm. That causes an imbalance where there is now more on the rock and sand than is stable with that amount in the water, and some comes off. After a period of time (minutes/hours/days), the tank water might be back to 0.18 ppm (a made up number, but it correctly illustrates the situation and is not exaggerated), and there is somewhat less on the rock and sand.

You'd need to repeat that 100% change many times to get to, say, 0.03 ppm.