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Just getting started cycling 2 identical systems, placed side by side.  Both have a 65 gal fish tank.  Following the Murray Haslam fishless cycling method (liquid seaweed), dosed the tanks 17 days ago.  My concern is that 10 days ago the nitrite reading in both tanks started rising rapidly, and has been off the scale that my API kit can measure for the last week.  This morning when I checked things, one tank was still at the high end, but the other has almost none (.25 ppm).  I repeated the test, thinking I had made a procedural error, but the results were the same.

Given that my tank volume is on the small side, is a swing this fast abnormal, or should I just wait a day or two and see if the other tank catches up?  I've held off on putting any fish in because of the high nitrite readings.

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Chuck,

I saw mine do that and it was fine.  I have a 20 gallon tank and 10 gallon grow bed.  For weeks I had high ammonia levels.  That went to 0 one day.  Nitrites rose to a lovely purple color 20+ ppm suddenly, after 3 weeks, went to 0.  A few days later Nitrates are, and remain at, 40 ppm and plants, fish are all doing great.  I see an occasional ammonia jump to .25 the it goes back down the next day.  Give it a few days, maybe increase flow for 24 hours if concerned but wait.  It is not abnormal for a sudden drop like that.  Watch Nitrates, Temp and pH.  I have stopped testing Nitrites but once a week now.  pH, ammonia and Nitrates I do daily, but will be going weekly on those for just pH daily.

Thanks for the quick reassurance.  The speed of the drop was rather disconcerting.

John Cubit said:

Chuck,

I saw mine do that and it was fine.  I have a 20 gallon tank and 10 gallon grow bed.  For weeks I had high ammonia levels.  That went to 0 one day.  Nitrites rose to a lovely purple color 20+ ppm suddenly, after 3 weeks, went to 0.  A few days later Nitrates are, and remain at, 40 ppm and plants, fish are all doing great.  I see an occasional ammonia jump to .25 the it goes back down the next day.  Give it a few days, maybe increase flow for 24 hours if concerned but wait.  It is not abnormal for a sudden drop like that.  Watch Nitrates, Temp and pH.  I have stopped testing Nitrites but once a week now.  pH, ammonia and Nitrates I do daily, but will be going weekly on those for just pH daily.

Hi Chuck,

The one with the .25 ppm will be ready for fish within a day or so, or to speed things along, exchange water(50%) between each FT to cycle both at the same time. If you want to remain with what you have now and want to wait to put fish in the both tanks when they are both fully cycled, you can dose the now almost cycled tank with small amounts of ammonia( careful not to exceed 2ppm) until the other one catches up and then introduce the fish to both of them. When i first cycled my system i had to wait for approx 10 days for the fish to arrive, so i continued to dose small amounts of Urea each day, maintaining the bacteria with a2ppm ammonia reading.

Chuck - What you experienced is pretty normal.  I would introduce a source of ammonia for two to three days after both tanks register 0ppm for nitrites.   This way you ensure that both of your bio-filters can successfully deal with all toxins in a 24 hour period.  Once you can do that for a few days I'd say it's safe to introduce fish.

second the theory. That is exactly how you are supposed to cycle. You dont want the nitrites and ammonia to linger in your tank too long. 

If your nitrites have suddenly disappeared without a trace of nitrates in your system, then you may have overwhelmed the API Nitrite test, causing it to come up as zero. If you have nitrates in your system, that means the bacteria are doing their job! Have you tested for nitrates?

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