Huge phosphate spike with turf algae

[Rick's Reefing Adventures]

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Hello!


I have a 15-gallon LPS tank that has been running for 2 years. Since the beginning, I’ve struggled to maintain phosphate levels above zero. To address this, I eventually connected phosphate to a doser and began feeding very heavily. This worked well—everything in the tank was happy and growing, with stable phosphate levels (tested weekly after water changes).


However, a few weeks ago, I tested my phosphate and found it had risen to 0.5 ppm. I immediately stopped dosing and reduced feeding. It seems my rocks and sand had accumulated phosphate over time, and after nearly 2 years, dosing was no longer necessary. I prefer to avoid chemical interventions, so I stuck with my normal routine to see if phosphate levels would drop naturally. Over the next three weeks, they did—down to 0.25 ppm—but this week, it’s risen slightly to 0.35 ppm.


Everything in the tank seems fine, except for a sudden outbreak of red turf algae. It appeared almost overnight, and within three days, it has covered everything. I’ve been scrubbing the algae and added more snails, but it’s not making much of a dent.


Currently, my nitrates test consistently around 11 ppm (fluctuating between 11.1 and 11.4). My challenge is to lower phosphate (PO4) without affecting nitrate (NO3). I’ve heard great things about ROWAPhos—does anyone have experience using it or advice on how to apply it effectively?


Tank Details:

• 15-gallon LPS tank
• Carbon (changed monthly)
• Skimmer (cleaned weekly)
• Weekly 10% water changes

Any suggestions are greatly appreciated!

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[UMALUM]

My experience with rowa is 1 its extremely messy and not easy to work with and 2 it will only be effective if used in a decent reactor. If a reactor is an option I would say go for it, if it's not I would think for your size tank weekly 5 gallon water changes might put a small dent in it without bottoming out nitrate. Unfortunately no matter which road you take the turf isn't gonna go anywhere soon and will take time without manual assistance. If the rock can be safely removed you could always strip it in a separate container. That amount of rock can be stripped in less than a day.

 

[Randy Holmes-Farley Verified Community Expert]

GFO will lower phosphate and not nitrate, but lowering phosphate will not likely solve the algae issue without also causing corals to suffer.

 

[Rick's Reefing Adventures]

GFO will lower phosphate and not nitrate, but lowering phosphate will not likely solve the algae issue without also causing corals to suffer.

yea the algae issue is an issue on it’s own, but the biggest cause is probably my phosphates so when that’s under control I can focus on the algae. What would you recommend as GFO? A lot of populair products are not availlable in the Netherlands…

My experience with rowa is 1 its extremely messy and not easy to work with and 2 it will only be effective if used in a decent reactor. If a reactor is an option I would say go for it, if it's not I would think for your size tank weekly 5 gallon water changes might put a small dent in it without bottoming out nitrate. Unfortunately no matter which road you take the turf isn't gonna go anywhere soon and will take time without manual assistance. If the rock can be safely removed you could always strip it in a separate container. That amount of rock can be stripped in less than a day.

Alright a reactor isn’t really my first choice but I will keep that in mind. 5G would be 1/3 of the tank. If that’s fine for like a month or so, I will try that. Removing the rock is sadly not doable due to some beginner cement mistakes at the start… If everything goes to plan, I will upgrade soon but I’d rather wait a bit longer so I’m not moving an already unstable tank.

 

[Randy Holmes-Farley Verified Community Expert]

I do not believe that most algae grow faster at 1 ppm phosphate than at 0.1 ppm phosphate. In both cases, they get all they need. Reducing nutrients is not typically a good way to reduce algae in a reef tank, despite the common assumption that it is.

 

[Rick's Reefing Adventures]

I do not believe that most algae grow faster at 1 ppm phosphate than at 0.1 ppm phosphate. In both cases, they get all they need. Reducing nutrients is not typically a good way to reduce algae in a reef tank, despite the common assumption that it is.

Alright good to know, still my PO4 levels are way out of line and do need to be lowered. I'm not sure why turf algae is a bad thing as I personally kinda like the look of it and it doesn't seem to affect my parameters too much. What GFO do you recommand?

 

[Pod_01]

Alright good to know, still my PO4 levels are way out of line and do need to be lowered.

Out of curiosity why do they need to be lowered??? Is there some type of target / reason?

If corals are:

Everything in the tank seems fine,

I would let PO4 do its thing, just don’t add any liquid PO4.

Or are things not looking fine?

Just for reference my PO4 is 0.15 - 0.35 and I just monitor.
Things look fine:

 

[Rick's Reefing Adventures]

Out of curiosity why do they need to be lowered??? Is there some type of target / reason?

If corals are:



I would let PO4 do its thing, just don’t add any liquid PO4.

Or are things not looking fine?

Just for reference my PO4 is 0.15 - 0.35 and I just monitor.
Things look fine:

Thanks! To be honest my tank is doing fine. Colors are good, growth rate is great. My goal used to be 0,06 to 0,1. I only have Euphyllia’s (and zoa’s / candycanes but they really don’t care). From what I’ve heard and read, if you go above 0,1 then it starts to get bad.

I stopped the liquid PO4 when I noticed I no longer needed to dose it so that has been a few weeks now.

The only reason I thought it was getting too bad was due to the turf algae. My glass barely has algae on it, and except the turf algae, my rocks and sand are also clean.

 

[UMALUM]

I think the war on nutrients was introduced to the hobby around the same time dry rock became a thing. People couldn't figure out why their beautiful white rock was covered in green hair algae and the phobia took hold. A lot of people also don't realize that a cuc and herbivores are a preventative and not a cure. I'm not sure my phosphate has ever been under 0.2 and I couldn't grow algae if I tried.

 

[Faurek]

Are your corals complaining? If yes lower the phos if not just let it be, you added phos, it's high but over months the corals will feed out of that phos, just don't use stuff that is high in phos like coral foods and fish pellets.

 

[Rick's Reefing Adventures]

Are your corals complaining? If yes lower the phos if not just let it be, you added phos, it's high but over months the corals will feed out of that phos, just don't use stuff that is high in phos like coral foods and fish pellets.

Corals are happy and growing. Yea I added just enough to stop it from bottoming out but apparently that wasn’t neccesary without me noticing. I’m feeding less and now only adding coral food once a week and my phosphates are slowly going down so I’ll keep doing what I’m doing as everything seems perfect.

 

[Faurek]

Corals are happy and growing. Yea I added just enough to stop it from bottoming out but apparently that wasn’t neccesary without me noticing. I’m feeding less and now only adding coral food once a week and my phosphates are slowly going down so I’ll keep doing what I’m doing as everything seems perfect.

Then it's all good let the corals have a go at it.

 

[rishma]

I do not believe that most algae grow faster at 1 ppm phosphate than at 0.1 ppm phosphate. In both cases, they get all they need. Reducing nutrients is not typically a good way to reduce algae in a reef tank, despite the common assumption that it is.

I just noticed this comment. The premise makes sense to me, but it actually does not match my experience. My current tank had moderate hair algae issues as I tried out various nutrient control methods. Phosphate was never as high a 1ppm. When I got phosphate consistently around 0.1 the algae issues really went away.

6 months ago hair algae seemed to be making a comeback, not bad but noticeable. I found my phosphate around 0.3. I got it back to around 0.1 and the algae slowly went away. I’ve stayed on top of it since then and the algae is really minimal. Small tank, minimal herbivores.

My target is 0.1. My corals don’t care if it’s a bit higher but my algae seems to.

I supposed it could be a spurious correlation, and I wouldn’t necessarily think it applies in general, but I’ve convinced myself it applies in my tank.

 

[Randy Holmes-Farley Verified Community Expert]

6 months ago hair algae seemed to be making a comeback, not bad but noticeable. I found my phosphate around 0.3. I got it back to around 0.1 and the algae slowly went away. I’ve stayed on top of it since then and the algae is really minimal. Small tank, minimal herbivores.

What did you do to lower phosphate that then led to reduced algae?

 

[rishma]

What did you do to lower phosphate that then led to reduced algae?

Carbon dosing. I had been carbon dosing a long time but had to up my dose. Had to add N sources to keep nitrate 5-10pm.

 

[rishma]

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Hello!


I have a 15-gallon LPS tank that has been running for 2 years. Since the beginning, I’ve struggled to maintain phosphate levels above zero. To address this, I eventually connected phosphate to a doser and began feeding very heavily. This worked well—everything in the tank was happy and growing, with stable phosphate levels (tested weekly after water changes).


However, a few weeks ago, I tested my phosphate and found it had risen to 0.5 ppm. I immediately stopped dosing and reduced feeding. It seems my rocks and sand had accumulated phosphate over time, and after nearly 2 years, dosing was no longer necessary. I prefer to avoid chemical interventions, so I stuck with my normal routine to see if phosphate levels would drop naturally. Over the next three weeks, they did—down to 0.25 ppm—but this week, it’s risen slightly to 0.35 ppm.


Everything in the tank seems fine, except for a sudden outbreak of red turf algae. It appeared almost overnight, and within three days, it has covered everything. I’ve been scrubbing the algae and added more snails, but it’s not making much of a dent.


Currently, my nitrates test consistently around 11 ppm (fluctuating between 11.1 and 11.4). My challenge is to lower phosphate (PO4) without affecting nitrate (NO3). I’ve heard great things about ROWAPhos—does anyone have experience using it or advice on how to apply it effectively?


Tank Details:

• 15-gallon LPS tank
• Carbon (changed monthly)
• Skimmer (cleaned weekly)
• Weekly 10% water changes

Any suggestions are greatly appreciated!

---

I keep a small tank too, a little bigger than yours. In the past I have had to use GFO and caused some issues. I had plenty of experience with GFO on larger systems, but it took some trial and error before I could get positive results on my small tank. I posted this thread which might help if you decide to go that way.
https://www.reef2reef.com/threads/a...owly-and-sustainably-lower-phosphate.1082298/

 

[Randy Holmes-Farley Verified Community Expert]

Carbon dosing. I had been carbon dosing a long time but had to up my dose. Had to add N sources to keep nitrate 5-10pm.

The carbon dosing may have consumed trace elements that then limited the algae.

 

[rishma]

The carbon dosing may have consumed trace elements that then limited the algae.

Ok. the most recent observation was an increase in dose after carbon doing a long time. I dose some trace with AFR but get an ICP so infrequently that I’ll never know.

The hair algae appeared, declined, appeared again, declined again and has not reappeared for months. The observation correlates with my management of phosphate levels with carbon dosing.

I have also had tanks that battle hair algae and have low measurable phosphate, so I understand it’s complex.