pH

There are few options:

• phDuino
• Oakton pH electrode, general purpose, BNC connector ($53) [TODO: ask for reading data from device]
• pH electrode designed to be used in aquarium (~$50) [TODO: ask for reading data from device]

Note

Check the innards of http://www.ebay.com/itm/Digital-PH-Meter-Tester-Water-LCD-Monitor-Pen-Aquarium-/160562316004?pt=LH_DefaultDomain_0&hash=item25624282e4 ($9) Althugh the best is to use off the shelf parts it might be useful to check if its possible to easily build one.