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Discussion Starter · #1 ·
Okay, once again, I'm totally lost in regard to my nutrient dosing.

On Sunday I do my big water change and dose N and P. Afterwards, I test and find that nitrates are around 30 ppm and phosphates are about 2 ppm. So far so good. Monday I dose micros and iron. Tuesday, out of curiosity I test before dosing more N and P. Nitrates are still 30 and phosphates are still 2, so I don't dose. This pattern continues for the rest of the week. Without me dosing any more N and P, my nitrates and phosphates remain at 30 and 2, respectively. They don't decrease at all, until I do my big water change on Sunday. Yet, my plants are growing noticeably throughout the week. Shouldn't N and P levels drop during the week as the plants use them for growth? Isn't that why EI calls for dosing them 3x per week? How are my plants growing if they are not using up the nutrients?

For the record all of my other water parameters are stable.
Ammonia = 0
Nitrites = 0
pH = 6.5 during CO2 injection
KH = 4.5 deg
GH = 10 deg
 

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Your plants are probably using the N and P more then you think. Remember fish poop and fish food also provide N and P. As long as your feeding your fish you are also supplying extra nutrients. That's probably why your levels are staying the same.
 

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Discussion Starter · #5 ·
Hobby grade test kits cannot be assumed to be accurate. Unless you verify their accuracy by testing water with known amounts of each of the nutrients in it you should treat your test kit results as if they are as likely to be wrong as right. See http://www.plantedtank.net/forums/showthread.php?t=83545 for one way to test your test kits.
I understand that the test kit readings might be skewed from what the actual ppm is. But still shouldn't I be able to detect a change in the reading over time? When I was cycling my tank I was able to watch the nitrates rise daily by testing. Sure, maybe what I thought was 30ppm was really 50ppm, but the point is I was able to see it change over time. Shouldn't I now be able to see a change (decrease) throughout the week as my plants consume nitrates/phosphates, granting that the test kit readings might be the true ppm?
 

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The readings would be assumed to go down with the plants using them. But the truth is, we may not see the test well enough to spot the small change. The change might be smaller than expected due to the fish food we use putting more in each week. It could be one of those odd cases where you are adding back almost the same amount that the plants are using so that at the end of the week, your readings may be the same.
 
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