Why did my nitrite and ammonia spike randomly?

[GallonsOfFun]

So my tank has been cycled for nearly a month now. I watched the water parameters like a hawk during and the two weeks following the cycle - everything as expected. I didn't check the water for about a week (so the last time I chekced it was 3 weeks after it cycled) and I went to check it today and I had 1ppm nitrite and .25ppm ammonia.

It's a 55 gallon and only has 4 tiger barbs in it currently, so the sudden spike is strange. I don't overfeed and I've been feeding the same amount. I do a 50% pwc every week.

The only thing I can think of is that I added 4 small plants and some duckweed, all of which has begun to die upon inspection today. Half the plants have been in there for 2 weeks and the other half for a week. Could their decomposition have caused the spike?

I have removed them and done a 50% water change.

 

[GallonsOfFun]

Anyone? Do you think it was the plants or do I have another issue at hand?

 

[coralbandit]

How did you cycle your tank?
IMO decaying plants can cause a nutrient spike but a cycled tank should not have registered ammonia or nitrite unless the dead plants were numerous.
How long ago did you add the barbs.

 

[GallonsOfFun]

Fish-in cycle. The barbs have been in there for about a month and a half - the bio load hasn't changed at all which is why I'm confused. I think I misread the ammonia reading, that was at 0 but the nitrite wasn't.

 

[Fel]

The bacteria that convert nitrite to nitrate take longer to grow than the ones that convert ammonia.
With only 4 fish your biofilter is probably still being established and the decaying plant matter was probably just enough to cause some nitrite to pile up.

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[GallonsOfFun]

Oh that makes sense. So the tank was fully cycled in the sense that the bacterial colony was large enough to deal with the 4 fish, but couldn't handle the added nitrite the decaying plant matter caused?

 

[Fel]

Right, once the planet material is removed nitrite should go back to normal

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