Nitrate low while phosphate high?

Jax15 · Jan 9, 2021

So my phosphate is reliable between 0.03-0.06, ranging between there month to month. However over the last 4-6 months, my nitrate has been going from 5, to 3, to 1... to now just barely detectable. Why do you think my phosphate can stay so consistent, but my nitrates are diminishing? Any ideas?

I don’t want my nitrates to get any lower, I liked them around 3-5. Feeding habits are unchanged, as is filtration. Only difference is the loss of 2 fish over that time, and added more corals. Tanks is 1.5 years and stable mixed reef. Few pics included for representation, most SPS is happy, LPS are hit/miss. Happy to provide full details and params if anyone thinks would be helpful - please ask! Thanks all

Daddy-o · Jan 9, 2021

You could add an amino acid like ME Polyp Extender. It will help raise the N while leaving the phosphate unchanged.
Cheers! Mark

Jax15 · Jan 10, 2021

Will give it a shot, thanks

Dkmoo · Jan 10, 2021

It's all due to what you are feeding and how it's being consumed. In a nutshell - the food you feed has a fixed N to P ratio (look up the redfield ratio, on avg it's around 16: 1 - is the ratio of phytoplankton - the basic lowest level of the food chain), but what the exact ratio of your feed is less relevant than how it's consumed). So, if how they are being consumed doesn't match exactly with how its put it, then eventually you'll end up with the inbalance that you are seeing now. The fact that your tank is showing this is actually a good sign - b/c it means that your tank is starting to mature and reach saturation, and whatever mechanism that was "hiding" this imbalance can no longer do so.

In general, algaes, corals, and zoox consume more N relative to P (something around 22:1), whereas pods, bacteria, and fish consume less N relative to P (varies, can range from 3:1 to up to 10:1). So what's happening in your tank is a couple of possibilities -

Possibility 1: What you fed in your tank has matched your consumption in more or less the same ratio in your tank's younger days where more of your biomass was made up of the second group above that consumes relatives more P. But now, as your corals have grown and you have more biodiversity, consumption is averaging more towards the N side. I also can't tell if what's on your sandbed is diatom or cyano but both cases are signs of imbalances of input vs consumption. Or,

Possibility 2: your consumption has always been a bit higher on the N side compared to what you fed, but the extra P has been absorbed inside of your rocks. Its not inconceivable that given your tank size, that it took 1.5 years for the rocks to fully saturate with P and now it's leaching the P back into your tank.

In both cases, this is a sign of a maturing tank. things you could do include changing your feed to a more balanced N to P ratio - ie - higher quality real foods or phyto. (cheap flakes tend to have more P), or as others suggested, dose amino to give it more N source. Since every organism consumes N and P in a fixed ratio, if you have no N left, P will also not be absorbed. adding Amino will help you soak up the P. Given enough time, your tank will reach an equilibrium based on the N to P ratio of your feed so focus on one that's higher on the N side (either by getting better food, or supplement with Amino) will ensure that the equilibrium is one that promotes coral growth.

More drastic measures include GFO but I generally don't like those b/c it's not a solution that will make your tank more stable and you will become increasingly reliant on it as your biomass increases.

Jax15 · Jan 11, 2021

Wow, thanks for the response dkmoo! Will try to get an action plan ready. I do have N I could just dose.. but the tank is happy, so seems dumb. Also there’s no Dinos or cyano on the sand, just a bad photo. Tank has only a tiny bit of cyano here and there on the rocks. Just the way it should be. My goal is to get the N back up and NP ratio back in balance before it bottoms out. Seems like feeding more + amino is a good starting point. Thx!

jda · Jan 11, 2021

The anoxic bacteria deeper in your sand and rock are using the nitrate and then turning it into nitrogen gas. This is normal.

This is the final stop of the nitrogen cycle... nitrate to nitrogen gas. There is plenty of science on it.

This is nothing to worry about. Just keep feeding your fish well and you will have plenty of nitrogen for your corals... it might not be in a form that you can test for. Although every tank has a breaking point, if you add more fish, then the anoxic bacteria populations would multiply and still keep your level low. The great thing about this is that the bacteria keep the nitrate low, but never too low to harm anything and the equilibrium keeps moving forward. My nitrate have been undetectable for a few decades thanks to the rock and sand, and only since ICP came along do I know that they are really .1 to .2. Photos in my re-build thread if anything thinks that this is a real problem.