Green Water Outdoor Tub Pond Testing With High Chlorine?

[Creedmoor Aquatics]

I have an outdoor 100 gallon tub that has been sitting idle for several years without any water changes or much activity beyond growing java moss, green water, and frogs. I wanted to add rice fish and wanted to know how acidic the water had gotten over time and if there were any minerals in it, so I used a test strip. 

As expected, the nitrates and nitrites were nil, the hardness was nil, and the pH was low.
What surprised me was the chlorine tested super high, returning a bright aquamarine.
I've no idea how there could be chlorine in the water, as our whole house is on rainwater and does not use chlorine. 
And if chlorine were added, it should have long, long, long since evaporated out.

Are there other chemicals which could affect the result?

[Lonkley]

Don't know, but if you had chlorine you wouldn't have green water and frogs. I would just accept it's an oddity and move on.

[Galabar]

I think some of those tubs can leach chlorine:

 

Edited June 1 by Galabar

[johnnyxxl]

On 6/1/2024 at 3:01 PM, Creedmoor Aquatics said:

I have an outdoor 100 gallon tub that has been sitting idle for several years without any water changes or much activity beyond growing java moss, green water, and frogs. I wanted to add rice fish and wanted to know how acidic the water had gotten over time and if there were any minerals in it, so I used a test strip. 

As expected, the nitrates and nitrites were nil, the hardness was nil, and the pH was low.
What surprised me was the chlorine tested super high, returning a bright aquamarine.
I've no idea how there could be chlorine in the water, as our whole house is on rainwater and does not use chlorine. 
And if chlorine were added, it should have long, long, long since evaporated out.

Are there other chemicals which could affect the result?

There are several threads on here about pond liners leeching chlorine into the water, it's used in the manufacturing of the plastic.