An EC meter (Electrical Conductivity) is a device that measures the electrical conductivity in the water and therefore is an indication, usually of salts in the system water.
EC measures salts, or total dissolved solids (TDS)
Almost all Hydroponic solutions are salts and therefore accurate measurements in a HYDROPONIC system can be read by noting EC readings. Hydroponic nutrients can be added to a Hydroponic system very accurately by using an EC meter.
Not so in an Aquaponics system. Our nutrients are not salts, so EC readings are only a very, very broad measurement of nutrients in an Aquaponics system and should not be taken too seriously, except when they do get too high, you should ask yourself, "Is there salt in the system? And, "where is it coming from?" Common salt can build up over time from various sources such as the use of low quality fish feed, or if you live in an area where ocean winds can carry salt into your system.
Generally, we do not like too much salt in our freshwater Aquaponics systems because food plants do not do well in salty water. Most freshwater species of fish can tolerate salt very well so it is not a problem for the fish. It is the plants that one would be concerned about.
An EC reading is also very useful if you need to have sick fish treated with salt in a hospital tank or quarantine tank where you would add salt to the water to treat the fish. In that instance a very accurate measurement of the salt added can be made with an EC reading device. Using salt to treat sick fish is one of the few ways we can treat fish that we plan to eat someday. It is not too desirable to use veterinarian type medications on fish we plan to eat someday.