Anyone please help me w/ these fert dosing miscalculations?

[cgcaver]

First, I started with these water readings from my tank:

PO4 = .5 (my tap water has .5ppm, so this is baseline for me)
NO3 = 0
K = Dont have a test kit, but Im assuming 0 because I didnt add any at all



Next, I did some reading to determine what I would need to add to hit my target levels. I used the directions on the bottles, and cross-referenced them with the info on rexgrigg.com's Fertilizers page (Rex's site has helped me incredibly). My target levels here are 10ppm NO3, and 1.0ppm PO4. You can check the formulas below at seachem.com

Here's what the bottles say to dose:

Nitrogen bottle says - .25 * 45 (gallons, my water size) * 10 (desired increase in ppm) = 112mL (amount needed to raise to that level)

Phosphorous bottle says - .8 * 45 (gallons, my water size) * .5 (desired increase in ppm) = 18mL (amount needed to raise to that level)

So I checked out Rex's site, and the dosing he posted for PO4 matched what the Flourish Phosphorous bottle said exactly. However, according to his caculation, for Flourish Nitrogen, I would only need 22.5mL to reach my target level of 10ppm (whereas the bottle formula works out to 112mL). This is a huge difference, and Im wondering where the discrepancies occur. However, I figured to do the smaller 22.5mL dose (since I trust Rex, and also, it's easier to add more if I needed to).




Then, I dosed my tank with:

22.5mL of Flourish Nitrogen
18mL of Flourish Phosphorus
60mL of Flourish Potassium
(And Flourish Trace, Flourish, and Flourish Iron - don't think this matters, but I did do it)


I waited 1:15 hours and then tested my water for a follow-up to my ferts dosing:

PO4 = 5.0 ?!?!? WTH?!
NO3 = 5.0
No2 = 0



Questions I know will be asked:

1) I use the same test kits Ive always used (Aquarium Pharmiceuticals), and they've always been reliable.
2) I am 100% sure I added the correct amounts. I was meticulously careful.
3) I have a 55gal tank with pressurized CO2. pH = 7.0, KH = 6, GH = 2. No chem buffers or other additives.



So... here's what I need help with guys: Why are my Nitrates HALF of what I want them to be, and why are my phosphates 5x MORE than what I want them to be?! I about had a heart attack when I took the Phosphates reading LOL Where did I go wrong? Bad math? Can anyone help me sort this out?

~ Thx all for your time

 

[travis simonson]

The calculations for dosing Seachem's ferts can be sort of arcane unless you find a reliable calculator, but you did an excellent job of hitting them about right. I checked your numbers on the APC Fertilator and it says those doses will give you 8.72 ppm NO3, 0.48 ppm PO4, and 20.04 ppm K - just about perfect for a high-light, CO2 driven planted tank.

Now there are only a limited number of possibilities when considering why your readings are off: 1] Your Flourish ferts have deteriorated and are no longer good, but this wouldn't account for elevated PO4 levels; 2] Your initial water conditions were not what you measured them to be, which leads to; 3] Your test kits are not giving you reliable readings. I know you say they are reliable but most kits aren't very reliable in the lower range reading (when NO3 is below 10-15 ppm, PO4 < 1.0 ppm, etc.). Try taking a water sample to your LFS or testing with a friends kits to get a second opinion. It's the only way to remove the test kits as a source for the problem. IME, only LaMotte or Hach test kits are highly reliable and most other brands only give you a ballpark estimate of levels at best.

Your reading of 5.0 ppm for NO3 is well within range of error for an AP kit so I wouldn't worry too much about it. The reading of 5.0 ppm for PO4 is the troubling one. You might consider getting a Seachem PO4 test kit to double check the readings. I've found Seachem's PO4 kit to be the most effective affordably priced test kit for measuring low levels of PO4. The Seachem kits also come with reference samples so you can benchmark them to verify their accuracy. If the Seachem kit also says 5.0 ppm PO4 then there is something else fishy going on

 

[shawmutt]

First thing I noticed is that Rex's and the bottle's Nitrogen measurements were the same. In parantheses on the bottle it says:

(for a desired nitrate increase use a factor of 0.05 instead of 0.25 in the formula)

Giving you the same measurement as Rex.

Second, check the Lot No. on your AP bottles. The last four digits represent the month and year the reagent was manufactured. Nitrate tests have a 3 year shelf-life, I couldn't find how long the PO4 test is good for.

Third, I found this little gem while having my own PO4 problems:

Phosphate tests. Test equipment that has not been soaked in dilute HCl (muriatic acid) and rinsed in distilled water, will be contaminated with sticky phosphate residues from the previous test. Or from PO4 in the tapwater it's been rinsed with. Professionals testing for phosphate don't let the test ampoules dry out even! Unless you are boiling your water sample with acid, your phosphate testing doesn't take into account any of the PO4 that is organically bound or built into organic molecules. Decomposers, like bacteria and micro fungi are constantly at work, releasing this PO4-- "mineralized"-- into the system once more. So phosphate could test higher two days from now, even if no PO4 were added to the aquarium system.

A good description of the care involved in executing a test for phosphates that is not misleading is at Brad Woods' Bald Eagle site Run through it once to see the various ways you may be contaminating your own aquarium PO4 tests.

My recommendation about amateur phosphate testing is, don't make decisions based on numbers you've got as test results.

Another confusing thing after rereading your post--your calculations are with 45 gallons, yet later on in your post, you say you have a 55 gallon. Which one is the typo?

 

[cgcaver]

its a 55g tank, with about 45g of water in it (due to rocks/substrate/plants)