Phosphate never seems to go down

[strich]

I bought a 2nd hand tank a few months ago, fish and all. You can see some shots in my build thread. Ever since I got the tank the nitrates and phosphates have been very high. The nitrates I'm fine with given the fish load, often sit around the 50s. Phosphates have always been some unknown value above 1. I have a Hanna ULR checker which maxes out at 0.9.
I just finished a month long hypo salinity protocol to test ick, and as part of that had to do huge water changes both going down at the start and back up at the end. I'd assumed phos would've plummeted by now through all that. But no, today it reads over 1 still!

Maybe the rock in this tank has built up a ton of phos?
What can I do to isolate the problem?

 

[Justfebreezeit]

Sounds to me the rock is likely the issue.

Choose a reduction method like gfo and reduce it slowly. Dropping po4 to fast can cause issues with corals.

Maybe someone has an idea

 

[get-salty]

did you reuse the rocks and sand? if yes, how were they handled/stored after break down from the previous owner.

 

[strich]

The sand is new. The rocks I kept wet and hopefully alive, and I also added some dry rock he had spare in buckets.

 

[get-salty]

The sand is new. The rocks I kept wet and hopefully alive, and I also added some dry rock he had spare in buckets.

Rocks kept wet at what temp?

 

[Fish Fan]

Phosphate binds to and can be released by your rocks, especially those new dry rocks. For that reason, doing like a 50% water change doesn't really lower the phosphate by 50% because the rock just releases more of it, and it goes back up right away.

You may likely need a strategy to pull some of the phosphate out until it reaches an equilibrium where the rocks are not releasing so much as to increase it so high. Something like a small amount of GFO, just be careful of not going too far and zeroing out the phosphate.

Good luck!

 

[EnterName]

As GFO and water changes were already mentioned:

I would recommend testing phosphate on a freshly made bucket of saltwater just to make sure you aren't introducing even more Phosphate through water changes.

Have you also tried a different test kit just to make sure the used hanna reagent is fine and the checker isn't reporting false measurements?

Macro algae can also help if you have space for an algae refugium or reactor.

 

[strich]

Thanks for the advice everyone. I'll definitely test the incoming salt water to be sure and validate the test kit.
I do already maintain some macro in the sump and I did buy a gfo reactor but never used it due to the hypo event. So I'll plug that in and start to slowly build that up.

And yep I def don't want to bottom it out. But at least getting it under 1 will be a good first step I think.