Chasing Phosphate

[flying4fish]

I've been reefing for about 6 years. Even with my Ph.D. in coral reef fish ecology, I'm always amazed out how difficult this hobby can be! For the better part of a year I've been fighting to lower phosphate. I tried carbon dosing and it dropped nitrate to zero without much impact on phosphate. I continued carbon dosing and added GFO. My phosphates began to come down, slowly. When I finally reached target levels of phosphate (.02-.05ppm), I discontinued carbon dosing and GFO. Phosphate continued to decline all the way to zero, and no matter how much I feed it remains zero. So I started dosing phosphate. If I check phosphate soon after dosing I can get a reading on my Hanna checker, but by the next morning it's zero. So I just put my phosphate solution in my auto-doser and started dosing every hour (1 ml of my stock solution which is 1090 ppm phosphate; 170L tank). I'm still not registering any phosphate.

I've read that the rocks and sand can be a phosphate sink; presumably that is why it took so long to come down. But how difficult is it to reach equilibrium? Will the substrate suddenly get saturated and my phosphate go through the roof again? Will I ever be able to stop dosing phosphate? I'm leaving on a 16 day trip in a week and I'm afraid what my happen while I'm away if I leave the dosing regime in place. I have a house sitter who can feed the fish, but she's not up to maintaining water chemistry!

 

[Miami Reef]

Will the substrate suddenly get saturated and my phosphate go through the roof again?

No. This won’t happen. The PO4 will bind to the substrate to reach equilibrium in the water. As the concentration goes up, the rocks will match the level in the water. It is an even and gradual process.

The only way phosphates will suddenly “shoot up” is if you stop testing and continue to dose for an extended period of time. Then if you test, you’ll see the phosphates much higher; the rocks didn‘t suddenly explode PO4.

I see no issue with dosing about 0.20ppm in one shot and removing the dosing pump. The rocks and sand should saturate within a day to find an equilibrium to your goal levels. Much quicker and it prevents phosphate starvation if you don’t feed enough.

 

[Miami Reef]

1 ml of my stock solution which is 1090 ppm phosphate; 170L tank)

I’m confused on this part. My math (which can totally be wrong) said that it would raise phosphates by 6.4ppm?

@Randy Holmes-Farley Did I do the math incorrectly? Adding 1mL of 1090ppm solution to a 170L tank would raise PO4 by how much?

 

[Mr. Acantho]

I use Brightwell NeoPhos. Follow the instructions and you should be able to below 0.05ppm for a while.

Cheers,
Mr. Acantho

 

[flying4fish]

An update, for those who may be interested. This morning I checked phosphate again and found that the auto-dosing of 1ml/hour of my stock solution of trisodium phosphate has now nudged my level to .039ppm. So the substrate must have gotten saturated with phosphate. The challenge now is to determine how much I'll need to keep dosing to keep phosphate in a healthy range. I'll cut it in half and see what happens in the next 24 hours.

 

[Randy Holmes-Farley]

I’m confused on this part. My math (which can totally be wrong) said that it would raise phosphates by 6.4ppm?

@Randy Holmes-Farley Did I do the math incorrectly? Adding 1mL of 1090ppm solution to a 170L tank would raise PO4 by how much?

I get 0.0065 ppm.

1090 mg/L = 1.1 mg/mL

1.1 mg in 170 L gives 0.0065 mg/L

 

[Miami Reef]

I get 0.0065 ppm.

1090 mg/L = 1.1 mg/mL

1.1 mg in 170 L gives 0.0065 mg/L

Thank you, Randy.

I understand it perfectly now!

 

[Randy Holmes-Farley]

Thank you, Randy.

I understand it perfectly now!

Happy to oblige.