Phosphate seems stuck high


New member
My tank has recovered nicely from a spike in salinity that caused some coral pain - looks great and remaining softies are happy but the phosphate level is stubbornly high at 10ppm - frequent water changes - ro di water tests negative for phosphate and I’m running a reactor containing a phosphate sponge - should i do anything else or just ignore at this point
First, I would get another test with a different kit to make sure your kit is accurate. Then, keep going with the GFO and water changes.
I agree - try a second test kit and/or take water to a friends for an second set of eyes.

How long has the phosphate been elevated?
What is total system volume?
How many and how large have the water changes been?
How much sand and how much rock? (pounds)
You would be amazed how much phosphate seems to leach into rocks and sand. Even with running material to drop phosphates it may take a long time.

I suggest what others suggest and try another test kit.

If you have a sump you may also look into lanthanum chloride to quickly drop phosphates. Just be sure you drip it in as it will severely harm corals if left in the unreacted form.
Lanthanum Chloride is terrible stuff. Any free lanthanum appear to accumulate on fish gills - especially Zebrasoma Tangs.

I was meticulous about dripping it into a triple layered filter sock arrangement with progressively smaller microns per sock into start of the sump.

It almost killed my tangs, no matter how slowly or infrequently i used it. So many people use this stuff with no regard to the harm/stress it causes. Once I noticed the issue at larger scale, I stopped for a long time. Then started with just a few drops A DAY at a po4 level of 2. I could see the fish stress within 1-2 days of starting. I would advise anybody against using this stuff.
That’s really good to know @BeanAnimal. I had gathered from others that treating phosphates higher than 0.5ppm was generally safe.
Also have seen people use iron gluconate but the amount needed would be prohibitive with phosphates at 10ppm.
That’s really good to know @BeanAnimal. I had gathered from others that treating phosphates higher than 0.5ppm was generally safe.
I assume that the more phosphate there is, the more likely free LaCl will have something to bind to. I had a very bad and repeatable experience and IMHO the stuff has no place around fish. Other people will feel differently.
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Total volume is 240 gallons. - prob 50-60lbs of live rock - I did three water changes of about 50 gallons in three days and phos is still 10ppm - fish and soft corals seems to be very happy though ! Running a reactor full of GFO with no effect
Your rock/substrate is at equilibrium with the phosphate. You pull out Po4 with the water change and more leached from the rock/substrate.

GFO will be a big help along with the (optional) water changes. Export via a refugium or turf scrubber is also efficient. In the end, you also need to understand how it is getting in also. Over feeding is usually the culprit, but change/top off is often also phosphate laden.
What is strange - at 10ppm my green button polyps and GsP are growing like crazy - also my leathers have never looked happier - and algae is actually at a minimum - don’t get it