Sudden chlorine spike?

[Josh W]

I've had a 120L tank cycling for about a week and a half (fishless). It's had trace amounts of ammonia / nitrite / nitrates as would be expected. Then suddenly this morning when I checked the water with a Tetra test strip I had a very high chlorine reading out of nowhere. I haven't done a water change in a week and there's been nothing new added into the tank.

Any ideas where this might have come from? I've tested multiple different strips with the same result, tested other tanks (which displayed no issues), then used Prime to knock the chlorine down and watched it vanish from the test... so I believe the readings are correct.

Has anyone had experience with chlorine showing up mysteriously this way?

[Simon]

Do you have wood in your tank?  Methyl chloride is a breakdown product of wood decay.

[Dawn T]

Are you seeing diatoms in the tank? I've read elsewhere that diatoms can react with whatever is in chlorine part of the test strip and give a false positive reading for chlorine. ?? The fact this is a new tank makes me wonder.

[Colu]

Do you treat your water when doing a water change with prime and have you tested your tap water for for chlorine

Edited January 17, 2021 by Colu

[Josh W]

Yes there are a few pieces of driftwood in the tank, perhaps that was it. I haven't seen it return and the driftwood has since been covered in a white furry coat of mold.

Thanks for the info, I'll research that a bit more. 

[Josh W]

Yep, always. In this instance I used Prime to knock out the chlorine right away. 

I'm wondering if this could have been chemical over-spray from my cleaners. The amount of chlorine that was showing up was far in excess what my tap water has in it. 

[Josh W]

No diatomes yet, the tank is very early into the cycle.