I had my big recirculating hydro going a few years ago. I over engineered it, but I learned a lot in the process, some of it heretical: <1> Air stones are not needed - fluming is better. <2> Yes, your medium *can* be too cold. <3> Chillers are easy to make. (see #2) A lot of the tuning I had to slog through on that system was removing the heat I was generating with my big assed pump, which was required due to having all the buckets and tubing. I'm thinking now of just building a containment (indoor pond) out of concrete block or fiberglass, laying boards with cutouts for pots across it, adding some small power heads to flume the water, and then adding a chiller to keep things around 55-60F. FNG @ ~1200 for the duration - stand em in a bucket at the end for final flush.