The aquarium must be completely sensorised to detect changes in water quality,of all sensors, to me there are two essential temperature and pH in the future I want to put conductivity sensors, nitrates and nitrites gauges.
To measure the temperature of the water I used a digital temperature sensor (DS18B20) this sensor can be purchased here. The DS18B20 provides 9 to 12-bit (configurable) temperature readings over a 1-Wire interface, this is also a waterproof sensor.
The pH sensor is difficult to find at good price to me it was very difficult to find a suitable probe, in the end I found looking on Ebay. The pH probe is an analog sensor which gives a signal as a function of the acid that is water, this was a big problem, because I had to create a board to adapt the signal, since the probe gives a very low voltage and has a large Impedance, this is the board
pH probe gives a voltage value between 0 and 0.5V depending on the pH of the pH range that can read go from 0 to 14. The conversion between voltage and pH is done by software.