Now you need to do your part. I gave you some links.
Here is what you do. You look at the links. You see how the sensors work.
Next you do some research on the concentration of chlorine you have to measure. A swimming pool should have lots. A fish tank almost none.
Dig out the range of measurements you need to make. The make some judgments about the sensors available.
Then you look at whether you are able to interface the sensors electronically.
The you try to imagine what type of program you would write to read the sensors -- then to display the sensors.
The you think about alarms.
For a pool -- you need an alarm if the concentration is too low.
For a fish tank you need an alarm if the concentration is too high.
by "Alarm", I mean the obvious -- flashing lights, beeping speakers, a Robot Screaming "Danger Will Robinson!" etc
Is this clear now?