Phosphate dosing - Am I doing something wrong?

[CaptainScuba]

Good morning everyone :)

I've been dosing anywhere from 9-12mL of NeoPhos DAILY (spread out in increments of 1-2mL every couple of hours). At this point I've been dosing it for 12 days and I've dosed a total of 75.6mL of NeoPhos (equating to a total of 2.72ppm of phosphate? if my math is right...)

The tank is a 10g Nuvo (7g of actual water volume), with 8lbs CaribSea Liferock, 10lbs Oolitic aragonite sand, and 500mL of Seachem Matrix biomedia. It has been running since 10 Dec 2026, so about 3 months.
Water quality as of yesterday evening (4 Mar 2026):
Salinity: 35ppt,
Temp: 75F
NO3: 25ppt
PO4: 0.03ppm
Calcium: 415ppm
Magnesium: 1260ppm
Alkalinity: 10.5dKH

Yesterday alone, I dosed 12mL of NeoPhos, and as of this morning my Salifert phosphate kit has phosphates undetectable and Hanna ULR Phosphate shows 0.03ppm.

Am I doing something wrong? Should I be dosing more all at once? Or is it best to continue spreading small amounts throughout the day?
I recently beat out a bloom of dinoflagellates and I'm trying to get my phosphate consistently closer to 0.1 in order to help keep them at bay. Based on my reading here, much of the phosphate is likely binding to the rock & sand. Will that eventually stop happening? I'm afraid that I'll keep dosing 12mL of NeoPhos daily (equaling about 0.4ppm phosphate in a tank my size) and I'll wake up one morning to a phosphate reading of 0.4ppm, way over-shooting my target. I also cant spend everyday for weeks/months dosing 2mL of solution into the tank periodically throughout the day. Work has been coincidentally slow the past few weeks so I've had the luxury to stay home and baby the tank.

Once this bottle of NeoPhos runs out, I've ordered a bottle of Sodium Phosphate Monobasic from Loudwolf, which I will mix with RO/DI for dosing in the future.

After all that the main questions are:

1. Is this an unusually large amount of phosphate to be dosing - is there any major risk involved?
2. Will the dosing eventually slow down as the rocks/sand begin to "fill" with phosphate?
3. Would dosing larger amounts all at once be better? Or is it best to continue doing small amounts periodically throughout the day?

 

[TheDuude]

I think your good dosing throughout the day.. but yes to answer your question in my experience it does take some time if your starting from dry rock and sand.

Also double check your test kit against another...

 

[CaptainScuba]

I think your good dosing throughout the day.. but yes to answer your question in my experience it does take some time if your starting from dry rock and sand.

Also double check your test kit against another...

I bought the Hanna ulr phosphate checker because I was worried that my Salifert phosphate was giving me false readings (always undetectable/0ppm). So in an attempt to save money/reagents; I've been testing with Salifert first, hoping that I'll get a reading. When it inevitably reads 0ppm, I double check with the Hanna kit to make sure I'm not "overdosing" the tank. So pretty much every day I've been testing with both with the same results. The salifert kit always reads 0 and the Hanna kit always reads 0.03ppm, except for one time that it got as high as 0.5ppm (boy did I get excited, it didnt last... lol)

I got worried that somehow both of my kits were reading incorrectly so I brought a sample to my LFS (they use Red Sea Pro test kits). They also got a result of 0ppm.

 

[TheDuude]

How are you running the matrix? I had to take mine offline since it was stripping nutrients down when I had my smaller reef . Powerfull stuff if in a reactor.

 

[Randy Holmes-Farley]

Neophos is fairly weak, and it can take far more than the calculated amount to hit a target concentration since much of what you dose will bind to calcium carbonate surfaces in the aquarium.

If the both tests have you below your target, up the dose until you get there.

 

[CaptainScuba]

How are you running the matrix? I had to take mine offline since it was stripping nutrients down when I had my smaller reef . Powerfully stuff if in a reactor.

It is in a little mesh baggy in the second chamber of my AIO. If you think it'd be worth removing, I could put it in a bucket of water change water with a small pump for a bit to see if it makes any difference?

 

[CaptainScuba]

Neophos is fairly weak, and it can take far more than the calculated amount to hit a target concentration since much of what you dose will bind to calcium carbonate surfaces in the aquarium.

If the both tests have you below your target, up the dose until you get there.

That's what I've gathered from my research so far (mostly you in various threads saying the same thing to oher people, sorry to make you repeat yourself). Do the calcium carbonate surfaces have a maximum capacity that they will reach and then stop binding? Or is there some reaction that is occurring that allows the calcium carbonate surfaces to bond with the phosphate indefinitely?

I'm sure I'll blow through this small bottle of NeoPhos and begin dosing with the sodium phosphate monobasic in no time.

 

[Randy Holmes-Farley]

It is in a little mesh baggy in the second chamber of my AIO. If you think it'd be worth removing, I could put it in a bucket of water change water with a small pump for a bit to see if it makes any difference?

I don't think that will be necessary or useful in the context of raising phosphate.

 

[Randy Holmes-Farley]

That's what I've gathered from my research so far (mostly you in various threads saying the same thing to oher people, sorry to make you repeat yourself). Do the calcium carbonate surfaces have a maximum capacity that they will reach and then stop binding? Or is there some reaction that is occurring that allows the calcium carbonate surfaces to bond with the phosphate indefinitely?

I'm sure I'll blow through this small bottle of NeoPhos and begin dosing with the sodium phosphate monobasic in no time.

Inorganic mineral surfaces such as calcium carbonate have an amount that will bind that is related to the amount of phosphate in the water. At 1 ppm phosphate, more will bind than at 0.1 ppm phosphate. Thus, as you try to raise phosphate, you will bind more and more.

The reverse is also true, making it much harder than expected to lower phosphate that is high, since some will come off as you try to lower phosphate.

 

[CaptainScuba]

Inorganic mineral surfaces such as calcium carbonate have an amount that will bind that is related to the amount of phosphate in the water. At 1 ppm phosphate, more will bind than at 0.1 ppm phosphate. Thus, as you try to raise phosphate, you will bind more and more.

The reverse is also true, making it much harder than expected to lower phosphate that is high, since some will come off as you try to lower phosphate.

Okay that makes a lot of sense. I appreciate the detail!

Based on what you're saying, does it make more sense to add a larger amount of NeoPhos at a single time? If 1mL NeoPhos is increasing my water's phosphate level to 0.03ppm, then it would be binding relatively quickly? If I dose 10mL at one time it will increase the phosphate level of the water to 0.3ppm and wont bind into the calcium carbonate surfaces so quickly?

Or will such a rapid change in phosphate levels likely stress out my tank inhabitants, meaning that I should continue dosing small amounts throughout the day, watching for levels to rise as the rock continues to bind more and more each dose.

 

[Randy Holmes-Farley]

When I dosed phosphate, I added 0.1 ppm at a time. I would not go greatly higher than that since it may get above your target.

 

[bluehippoguy]

FYI it says on bottle you should not raise it more than .04 ppm in a 24 hr period.

 

[Randy Holmes-Farley]

It's fine to go slow, but if you add 0.04 ppm, you may see little actual effect as much of that may bind the rock and sand.

 

[BriDroid]

I had the same issues with my nano that was started with dry rock. You have to dose a TON to get the rocks and sand from absorbing it. Just keep at it, you'll get there!

 

[CaptainScuba]

I've been dosing anywhere from 10-15mL of NeoPhos daily and finally today I have a reading of 0.07ppm. I am going to slow it down and see if it begins to stabilize or if I will need to continue pushing it to stay where I want it

 

[Randy Holmes-Farley]

I've been dosing anywhere from 10-15mL of NeoPhos daily and finally today I have a reading of 0.07ppm. I am going to slow it down and see if it begins to stabilize or if I will need to continue pushing it to stay where I want it

Sounds good. :)

 

[Jack_L]

I've been dosing anywhere from 10-15mL of NeoPhos daily and finally today I have a reading of 0.07ppm. I am going to slow it down and see if it begins to stabilize or if I will need to continue pushing it to stay where I want it

hi found this as i'm trying to do what you did. my PO4 is zero, and dinos abound, snails are dying, and i've lost and SPS that was doing great. so if i follow you, you dosed daily for 20 days before you got a reading is that right?

 

[CaptainScuba]

hi found this as i'm trying to do what you did. my PO4 is zero, and dinos abound, snails are dying, and i've lost and SPS that was doing great. so if i follow you, you dosed daily for 20 days before you got a reading is that right?

20 days before i got any significant reading overnight (I would dose throughout the day, and test first thing in the morning after things would settle overnight. It probably took nearly another 20 days to see that reading hold without dosing. I check it weekly and last week it was sitting at 0.07ppm and I've only been dosing a tiny bit once a week. My tank is fallow right now bc of ich outbreak, so I'm sure once i add fish back it will be even more stable, or growing.

 

[Jack_L]

thanks for reply scuba, sorry to hear of the ich. i've done fresh for 40 years, 6 months into salt, and now i understand why i see so much salt tank stuff on marketplace. my DIY solution is much stronger than neophos, and i'm dosing 3x a day. today i finally saw a reading after a dose, but it was gone 4 hrs later. it seems like the corals can sense the phosphate dose too, the pull into the skeleton TIGHT after i dose. i'm losing corals too, but i'm going to keep dosing as it seems from my reading and talking to the couple humans IRL that keep reef, my zero phosphate is the root issue