To start with something, you can evaluate different sensors, for example looking on SparkFun you can find a PH sensor (SEN-10972) and temperature sensor (SEN-11050). I've seen some chlorine sensors around, for example Chlorine, disinfection, try to see which type is close to the probes you already use in the pool.
Doing a quick search on Google i found a document where a project similar to yours has been discussed briefly: https://engineering.purdue.edu/ece477/Webs/S11-Grp15/hw/hw3ug.doc