Live phytoplankton and PO4 testing

Reef Jedi · Oct 13, 2026

Hey!

I’m a little confused and need some help as I’m newer to using live phytoplankton. I recently did a test yesterday for the first time in 6 months of no3 and po4. No3 was 33ppm but po4 was .3. I was surprised to see po4 that high and proceeded to do a 40% wc last night. After the wc I added a very small splash of phyto about 20ml. Today I tested to see what levels I was left with and no3 is 24.1 and po4 was .411. I figured something had to be wrong with my test so I put a new battery in just incase and re ran it. The second result was po4 at .392. All tests are ran with Hanna checkers.

I would have figured the w/c would have reduced that number by about 40% however it did not. The no3 reacted as expected with the w/c. So the only thing I can think of is maybe the algae barn live phytoplankton is adding po4 back in?

I’m not too adverse with po4 and live phytoplankton so I really need others with this experience to chime in.

Here’s some tank shots

Other levels incase I’m asked:
Temp: 78
Salt: 35ppm
Alk: 8.3
CA: 455
Mg: 1350
Ph - currently 8.39

Dan_P · Oct 14, 2026

Try testing the concentration of PO4 in the phyto additive. Measure the PO4 in a sample of aquarium water before and after adding the phyto

Submerge · Oct 14, 2026

Can you measure phos in phyto on Hanna? Seems the color of the phyto would render any result useless. Or at least skewed.

Fish Fan · Oct 14, 2026

What's up, Jedi!

A couple thoughts: Phosphate is bound by and released from rocks and sand. They can often sequester a ton of phosphate, and when you do a water change and remove what's in the water, the rocks and sand just release more phosphate back into the water. For this reason, doing a 40% water change doesn't really cut the phosphate by 40%, like it would for nitrate, for example. This continues for some time until the phosphate bound up in the rocks reaches an equilibrium. I'm in an opposite situation where my rocks and sponging up phosphate, and I have to dose it to keep in above zero.

And yes, you could be adding phosphate from the fertilizer used to culture the phyto. As Dan mentioned, check the phyto water to see if that's high in phosphate.

If your problem is new, it could be the fertilizer, but if you've seen this for some time, I think it's your rocks.

Ossss, Jedi!

Reef Jedi · Oct 14, 2026

Fish Fan said:
What's up, Jedi!

A couple thoughts: Phosphate is bound by and released from rocks and sand. They can often sequester a ton of phosphate, and when you do a water change and remove what's in the water, the rocks and sand just release more phosphate back into the water. For this reason, doing a 40% water change doesn't really cut the phosphate by 40%, like it would for nitrate, for example. This continues for some time until the phosphate bound up in the rocks reaches an equilibrium. I'm in an opposite situation where my rocks and sponging up phosphate, and I have to dose it to keep in above zero.

And yes, you could be adding phosphate from the fertilizer used to culture the phyto. As Dan mentioned, check the phyto water to see if that's high in phosphate.

If your problem is new, it could be the fertilizer, but if you've seen this for some time, I think it's your rocks.

Ossss, Jedi!

This is interesting! The Rocks have been in the tank for about 2 years and were from TBS. I don't ever remember po4 being this high, most I've ever seen was .1 and that was a year ago. I did have a bunch of issues at first raising po4 like you and ended up bottoming out with a Dino outbreak. I beat that last year and tank has been great since.

Maybe it really is adding the Live photo from algae barn. I would have expected it to decrease on the w/c if it was from phyto. the level is so high, Im not sure if I trust the hanna checker. I thought about running a test on the RODI water to see if there's any in that but not sure if the checker will test that. I was planning on doing maybe another 20% wc to see if that reduces any more? Thoughts?

I was just taken back by po4 going higher and not reducing like I would have expected.

OSSS!