Fishless cycle mystery: Nitrates went down?!?

[bob965]

I have been conducting a fishless cycle for the past 20 days or so. I added ammonia for a 2ppm concentration and waited, adding more as necessary to maintain 2ppm. I started detecting nitrites after about 2 weeks. Detected nitrates 1 or 2 days after that. Nitrites increased over the next couple days until it was at the highest level my kit would read. Nitrates increased over the next 4 days up until yesterday, when it was 60ppm. I was continuing to add ammonia each day in the same amount necessary to attain 2ppm, and it would be consumed overnight and would read 0ppm by the time I tested the next day. Then, when I tested today, ammonia was 0ppm, nitrites were off the scale high, both as expected, but the nitrates had gone back down to 5ppm. Is something going wrong with my cycle? Nitrates shouldn't decrease unless I change water, so I'm wondering why I'm showing a decrease.

Facts: tank volume-75g, temp-80, pH-7.9, test kit is an API master freshwater kit, pretty new. I conducted the test twice just to make sure. it read 5ppm both times. filter has been running continuously. Tank contains gravel, driftwood, fake plants, and an airstone. I haven't done any water changes.

Any help would be appreciated. Thanks.

 

[1better]

Are you sure it was the nitrate and not the nitrite that came back down? The nitrite level should come down after a while so that would be expected.

 

[bob965]

No, it was definitely nitrates that went down. Nitrites are still off the scale high (>5ppm)

 

[CHOMPERS]

Is there water flow through your gravel? Also, how deep is it and what is the grain size?

I have a theory but I wouldn't expect it to happen so quickly in your tank. I think your gravel may be acting like a Deep Sand Bed (DSB) that is common to the Saltwater community.

I have a 75g tank that consumes nitrates (not totally though). Its gravel bed is 4-5" deep and is composed of gravel 1/4" to 3/8". I have UGF plates without flow through them so they are acting as a plenum.

 

[bob965]

There is no water flow through the gravel, and it's less than an inch deep. The grain size is fairly large, as it is hardware store-type pea gravel.

 

[JakeH]

You lost 55ppm of Nitrate overnight with no WC's?! Impressive, but I suspect Pilot Error somewhere. Maybe you didnt shake Nitrate test bottle #2 long enough?

 

[bob965]

You lost 55ppm of Nitrate overnight with no WC's?! Impressive, but I suspect Pilot Error somewhere. Maybe you didnt shake Nitrate test bottle #2 long enough?

I agree that operator error seems the only reasonable explaination. But I'm pretty positive I did everything correctly. I used to be a nuclear power plant operator in the navy, so I'm pretty meticulous when it comes to exact compliance with written procedures. I even went so far as to set a timer for the 30 seconds of shaking bottle #2, and 1 minute of shaking the sample, and then 5 minutes to develop.

The sample was definitely a deep red (approx 60ppm) one day, and a light amber color (approx 5ppm) the next. I tested again tonight, and got the same result (5ppm), then I tried something... I used mostly clean water, mixed with a few drops of the tank water, and got a result of 20ppm?!?

Is it possible that the actual concentration of nitrates in the tank is so high, that it's giving me some kind of funky color result that just happens to look similar to the light amber color?

 

[Aeroxide]

Algae and plants filter nitrate among other things out of the water. I'm not sure if this is possible, but maybe the levels of nitrobacter aren't high enough yet and can't produce nitrate as fast as the algae can filter it out of the water. This would make sense if you got a large algae bloom or added lots of plants right about the time the nitrate levels dropped.

 

[pacu mom]

When we fishless cycled our tank, it took a long time for the nitrites to drop (it seemed like forever). Once the nitrites drop, you will have a huge spike in nitrates. It took us over 30 days to cycle our 300 gallon tank. Getting the nitrites to zero out is the longest phase of fishless cycling. I wouldn't worry about the low nitrates now, especially with high nitrites. It takes longer to colonize the bacteria that deal with the nitrites. Once established, they are very hardy. We have had a few small ammonia spikes following gross overfeeding or power outages, but we have never ever had a nitrite reading once the tank was cycled.

Be patient, you're almost there. I'm sure that you're aware that the nitrate test is extremely technique sensitive and requires vigorous shaking of the bottles and the test vial for the full time to get accurate results. Anything less than following the instructions explicitly will result in inaccurate readings.

I'd say that if your nitrites zero out and you still have a low nitrate reading, then something is amiss. I don't think that's going to happen, though.

 

[JakeH]

Well, if the test is repeatedly giving you the lower reading, I'd maybe pick up a pack of test strips or something, just to check it against. Either way, I say keep up your cycling for a while longer just to be safe...