Reef Aquarium Salinity Sensor

I am busy putting together a reef aquarium controller using an Arduino Mega. I am looking for a sensor which can measure the salinity (salt content) of the water. I have heard that one can measure the conductivity of the water to determine the salinity?

Any ideas on what I could use or if anyone knows of sensors available that won't break the bank?

Conductivity is affected by temperature, but in an aquarium the allowed temperature range will be so small that no special compensation may be required.

Use non-intrusive conductivity probes, or at least AC driven probes to minimize electrolysis.