Reef nutrition

N and P dosing

MarAquatic said:
Do you guys think algeq also grows with phos? Should we be dosing Both?

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Phosphate a usually isn’t a problem for me. A lot of it comes my feeding of pellets and reef chili. I think Oyster Feast has a significant amount of phosphates as well.
 
I think I over did my phos dosage
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I sometimes dose nitrate (BW Neo-Nitro) or carbon (NOPOX). Like most people’s tanks, mine is carbon-limited. In general my P is borderline high and chronic low P isn’t a problem for me. N is more variable and sometimes my N bottoms out. I have never had to dose phosphate, feeding a bit more and/or removing GFO is much better in my opinion. Plus my P is rarely too low.

I’m targeting around 1 ppm NO3 and 0.1 ppm PO4. I’m more fine with N going higher than I am with P going higher.

I don’t do any of this by doser, I hand dose based on testing and observation. If I need to dose N, I dose enough to raise it by 1 ppm at a time. If I need to dose C, I follow the dosing instructions on the bottle of the NOPOX.

So for me:
  • If both N and P are too high, I dose carbon
  • If N is too low and P is fine (common), I dose nitrate
  • If N is fine and P is too low, (rare), I feed a little more, and/or take GFO offline
  • If both N and P are too low, I feed a little more
  • If N is fine and P is too high (common), I dose nitrate and carbon
  • If N is too high and P is fine (rare), I dose carbon, maybe feed more and/or take GFO offline if P drops
 
Can you all help me sanity check something? If you dose potassium nitrate, KNO3 (?), will the potassium concentration added be the same as the nitrate concentration added? So, even if you added effectively .5ppm nitrate per day, your potassium would not increase more than .5ppm per day, right?
 
Chromis said:
Can you all help me sanity check something? If you dose potassium nitrate, KNO3 (?), will the potassium concentration added be the same as the nitrate concentration added? So, even if you added effectively .5ppm nitrate per day, your potassium would not increase more than .5ppm per day, right?
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Yes that is approximately correct.

At finer grain detail though, ppm in our use is by weight, as mg/liter, not molar concentration. So the molecular weights come into the calculation.
K= 39.1 g/mol
NO3 = 62.0 g/mol
KNO3 = 101.1 g/mol

So for each 1 mg of KNO3 you add, you’ll be adding about 0.4 mg K and 0.6 mg NO3. Which means if you add enough KNO3 to raise NO3 0.5 ppm you are raising K by 0.33 ppm.

Since K is about 400 ppm in our tanks, and a wide range is acceptable, raising by 0.33 ppm even repeatedly is probably negligible.
 
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