Hi,

I assume somebody has answered you already, but just in case...

The main difference is that when you have the drain at the top, the nutrients and other chemical elements will culminate in the buckets and won't drain out. So if you were to measure the EC, or any other levels in a water reservoir outside of the bucket, the values would be way off from the reality.

As for the nutrient measurement, you can just stick to the EC. But when adding the nutrients to the solution, do a quick research on what's best for the particular plat you want to grow.

Hope that helps.