EC and ppm are basically the same. They will each give you an idea of how much fert is in the water. You're right about conductivity and ppm is parts per million. I don't know what a good EC numbers are like but ppm range is usually 500-750 when their small and up to 1500 when their big. You add fert to keep close to those numbers throughout the grow. I'd paint the lid black if I were you, but that's container's wrong. If you do this right, 8 plants will quickly outgrow such a small container. For 8 plants you can use 5 gallon buckets seperately or put two or three plants in large tubs. 8 plants done right in DWC can easily take over a small bedroom if done right. But you've go to do it right, use the right fert, don't overdo the nutes, average temp 75F, give airflow, lots of air to the water with your pumps. Best of luck. Any questions, just ask.