Aquaponic Gardening

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Hi All!

I recently posted a question about a cycling issue I was having, but I've now noticed that my nitrate level has decreased from 10ppm to 5 ppm within a day or so. There were no major changes to my system during this time (no water changes, huge pH changes, addition of anything, etc.). It is a 100 gallon system with a pH between 7.0-7.6, hydroton clay pebbles, temperature ~89F, currently no fish in the tank (in the midst of a fishless cycle), NO PLANTS (yet) and NO ALGAE. Also note that the ammonia levels have been dropping steadily (from ~3ppm to .5ppm in a day) and the nitrite levels are off the chart (5+) but are not so high that they make the API test tube turn blue-grey, which happened to me before and I did an entire water change that complicated things and will not likely do again!

 

My suspicion is that there is denitrifying bacteria converting nitrate into molecular nitrogen. I say this because there's about a 1/2"-1" (or so) pool of water that doesn't drain out of the grow bed because the bell siphon's hole is at that level and that is the height at which the siphon breaks. The denitrifying bacteria grow in anaerobic environment (such as the stagnant water on the bottom of the grow bed that never gets siphoned). I also find it hard to believe that this is the reason for the decrease in nitrates because how could a volume of water that is not more than 10 gallons (which is the amount that is at the bottom of the grow bed) have enough anaerobic bacteria to convert 100 gallons worth of 5ppm of nitrate into diatomic nitrogen in one day--it just doesn't seem like there'd be enough of the denitrifying bacteria available to do this. Also, isn't a level of un-siphoned water in the grow bed a common occurrence in aquaponic systems because they are a largely unavoidable
consequence of having a bell siphon, which are pretty ubiquitous in aquaponics; so wouldn't this problem be encountered more frequently if this actually was the cause?

My questions are: is my explanation likely the actual cause of the drop in nitrate levels (I don't think so for the above reasons)? If so, how can this problem be corrected, or does it not even need to be corrected and the cycle will take its course despite this issue. Also, please provide any other explanations that may account for this observation.



Thanks!



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Perhaps the cause could be as simple as an imperfect test or two?  Perhaps it could be a bit of denitrification going on but since the water mixes in the bed each time the bed floods and drains it isn't like that bottom bit of water that doesn't leave the bed is the same water each time.

 

To tell the truth, I wouldn't worry to much about it at the moment.  Keep on as you are and watch to see if it is a trend or if perhaps the earlier high nitrate reading was maybe inaccurate or this newer reading was just an odd ball low reading.  It is also possible algae could be growing but you just can't see it yet.  So many possibilities but I don't think any of them are too alarming for now.

Quick and informative, as always!!

 

Thanks a bunch.

TCLynx said:

Perhaps the cause could be as simple as an imperfect test or two?  Perhaps it could be a bit of denitrification going on but since the water mixes in the bed each time the bed floods and drains it isn't like that bottom bit of water that doesn't leave the bed is the same water each time.

 

To tell the truth, I wouldn't worry to much about it at the moment.  Keep on as you are and watch to see if it is a trend or if perhaps the earlier high nitrate reading was maybe inaccurate or this newer reading was just an odd ball low reading.  It is also possible algae could be growing but you just can't see it yet.  So many possibilities but I don't think any of them are too alarming for now.

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