Chlorine (Cl), measured in ppm, is a gas which has been added to tap water to control harmful bacteria. City provided tap water is normally found to have 0.5 - 3.0 ppm but higher surges are sometimes observed. Some city water supplies can still be found that either do not require chlorination or may have the chlorine removed before the water is distributed. This would not be of concern to those who take their tap water directly from a private well. Droplet and pill test kits are available. Recommended test kit range 0 - 4 ppm. A chlorine test kit is not considered necessary for the average pond.
Acceptable concentration 0
Chlorine is a quick killer in fairly low concentrations (less than 0.5 ppm). Even in very small concentrations, it burns the edges of the gills with long term after effects. It also can be deadly to the bio-converter bacteria.
In an open container, water will release about 1/4 of the chlorine concentration per day to the air . Water that has set in an open container for a week or just for a couple days if aerated, is normally safe to use or better yet, pre-treat tap water with one of the commercial chemical products. Follow the manufacturer's directions (Or make your own).