weird nitrate reading while cycling

[janderson]

Here I am again...

This is about tank #3 that I'm currently cycling (MTS confirmed, I fear): it's a 37G, on day 6 of a fishless cycle. So far, ammonia has disppeared, nitrite has been holding steady at 2 for 4 days now and nitrate readings went as follows: 0-0-5-10-20-40 and today: 5! I don't understand...

I repeated the test because I thought I might have screwed up, but with the same result. Testkit used is "Freshwater Master Test Kit" by Aquarium Pharmaceuticals.

The other two tanks I cycled lately showed steadily increasing nitrate readings, and I'm following the exact same protocol. They both took about two weeks to cycle, plus two or three extra days to make sure ammonia and nitrite indeed stayed at 0. The only difference this time around is that the current tank has live plants in it (I got a freebie bag full of plants and needed to put them somewhere). Are the plants utilizing the nitrates? And are they interfering with the cycle? Please tell me I'm still on track...

Thanks for your help!

 

[RTR]

Yes, healthy and actively growing live plants use nitrates. Perhaps the utilization lag was just time for them to re-establish in your tank and start actively metabolizing. The disadvantage is that they use ammonia as well, so it is hard to judge how competent the bacterial biofilter is at this point. When ammonia and nitrite are both zero with 12-24 hours after redosing ammonia, you have to consider the tank fish-ready, but the percentage of that readiness due to the plants versus that due to the biofilter cannot be easily determined.

Continuing to add ammonia is a bit of an algae hazard in any case, as the algae will establish on the ammonia just as the plants do.

 

[janderson]

Thanks for the reply,

I'm glad there is a reasonable exlanation for the reduced nitrate readings. The plants are indeed starting to grow, so I guess it's time to add a regular fertilizing routine as well.

However, I'm a little confused by your comments on ammonia. Forgive the noob train of thought, but why does it matter how ammonia is used up in a tank? As long as something is using up a given amount of ammonia per day, shouldn't the fish be fine either way?

I understand the bacterial process (ammonia -> nitrite -> nitrate, the latter needs to be reduced through water changes), but just how do plants use ammonia? Are they, too, converting it into nitrite, thereby making a "standard" sized bacterial colony necessary for the reduction to nitrate? In that case my current setup would simply create a somewhat smaller bacterial colony for the the ammonia -> nitrite conversion with the plants doing the rest of the job. Or are plants effectively deleting ammonia from the tank, thereby making the tank stable for fish with acomparatively smaller bacterial colony altogether?

And about algae - so far I've been lucky, but either way: right now the only way to get through the cycle is adding ammonia, no?

 

[RTR]

Oops! Sorry, no background given, nor refs. Plants take in ammonia preferentially as their nitrogen source (land plants do this as well as the aquatics), but so do algae. Higher plants can and do also take in nitrate, but reduce this enzmatically back to ammonium ion. There is a small energy cost involved in that process, but not a major expenditure. Nitrite is not absorbed or used by plants, only the more reduced form of ammonium or the more oxidized form of nitrate. The intermediate nitrite form is apparently too toxic for plants or animals to handle.

We do not add ammonia/ammonium ion routinely for the plants due to ammonia's toxicity for the fish. Ammonia in water exists in equilibrium with ammonium ion, which is non-toxic. The balance between the two forms (NH3 or NH4+) is determined by pH and temperature. Thus, adding ammonium ion would result in potential or actual toxicty to our fish and inverts such as shrimp (snails are not very sensitive) as part of the addition would shift to the dissolved ammonia form. Algae can also use ammonium easily, nitrate less easily than do higher plants, so dosing ammonia is counterproductive with higher plants, at least potentially.

Nitrate in water is pretty stable unless there is provision made for its reduction back to gaseous N2, nitrogen gas (plenums, coil denitrators, etc.), or if the tank has healthy plants.

For your situation specifically, the nitrite present from bacterial processing of ammonia is going to require bacterial processing to nitrate, as neither plants nor algae will take it up. After bacterial conversion to nitrate it will again be available to the plants. I think I would skip a day's ammonia dosing and see if the existing nitrite gets cleared. What titer are you pinpointing for the daily addition now?

 

[janderson]

thanks, I think I understand a little better now

However,

What titer are you pinpointing for the daily addition now?

- I'm not sure I understand. Are you asking how much ammonia I'm adding daily? 15 drops, which happens to be one pipette full. It took me double that amount to initially get the ammonia concentration to 4 ppm at the very beginning of my cycle.

 

[RTR]

If you are adding to 2ppm daily - volume additions don't give any info really, there is too much local variation in the strength of ammonia solutions - that is fine, but I still would try to skip a day's addition and see what happens to the ammonia/nitrite levels.