We do not recommend using our sensors for long-term data collection. They are intended for use in a classroom during a lab period. Alternative suppliers are listed at:
http://www.vernier.com/company/terms-and-conditions/industrial-providers/

There are two main issues when trying to monitor long-term water quality:
1) They are not designed for long term deployment, and
2) These sensors, in particular can interfere with one another in an aqueous solution.

If you are intent on attempting this anyway, your best bet would be to use the Arduino to lift sensors out of the water when not being used and then place them in the system one at at time. You may need to move the pH sensor to a storage solution when not being used.
For more information related to sensor interference please refer to Will I get interference when I put two sensors in the same solution?

See also:
Arduino and SparkFun RedBoard Troubleshooting and FAQs