How'd My Tank Partially Un-cycle?

[eschaton]

I did a complete test of my water readings today, and for the first time in ages, I had ammonia in my 40-gallon planted of around 0.75. No one in the tank was acting funny, and I immediately did a 40% water change, but this confuses me, because the tank has been properly cycled for months, and just last week had perfect readings.

No new stock were added to the tank, in fact a fish was taken out (fin-nipper returned to store). The filter media was not changed, nor exposed to air. My filters didn't stop or clog up...flow rate has been pretty consistent, actually improving on one of the filters as I cleaned out some gunk from the tube (which should have had minimal to no impact on filter bacteria. There was some extra decaying plant matter in the tank though. I decided to try and feed my fish soybeans as I had some spare edamame laying about, and I threw in two to see what would happen. After a day, no one was doing much, so I figured I'd leave em in awhile to soften. Two soybeans shouldn't be enough for a mini-cycle however.

The one thing I did do this week was a major clean-out of my plants. One corner of my tank had tons of weedy, unattractive plants like water wisteria and parrot's feather. I yanked essentially all of them out and replanted the area with some more low-growers.

I'm left with the hypothesis that removing a significant amount of plant matter from the tanks at once destroys the cycle. Or, more aptly, that since plants prefer to directly use ammonia, there is no real cycle to speak of in a thriving planted tank, and quick removal of a major portion of the plant biomass will make the tank appear uncycled.

Anyone have similar experiences?

 

[nukeonekitty]

My first idea may seem dumb, but nonetheless, is possible. Maybe a fish died and you don't see it? That could cause an ammonia spike. Also, not doing water changes and over feeding the fish can create ammonia spikes.

 

[shellyboylan]

maybe uprooting the plants disturbed waste in the gravel, which dispersed into the water causing a spike?

 

[eschaton]

All fish seem accounted for. The only possible dead fish would be a Kuhli Loach or one of the Otos, since I see them so infrequently.

Water changes have been weekly, and feeding hasn't changed recently. I also have a sand bed, not a gravel bed. While some of the plants were deeply-rooted, less waste gets into the lower regions of the sand, so I'm not sure it would make a big difference.

 

[nukeonekitty]

I don't know if this is right, but I recall hearing something about sand being able to trap toxins and form pouches if the sand is not disturbed... maybe that's an option.

 

[SuperColey1]

The sand can form anaerobic pockets, but this shouldn't happen in a planted tank because the plants use this organic matter and take it through their roots, thus cancelling out the problem.

Maybe you left some dead roots or other organic matter behind and this caused the problem since there were now no roots to negate the problem?

Plus you shouldn't have a problem on anaerobic pockets with Kuhli loaches as tehy turn the sand over for you.

I can only assume that as plants will use ammonia that there wasn't a large enough colony built up in the filter media, therefore causing a small mini cycle when you took all the plants out.