The new system is nearly entirely built. I have learned a lot of lessons, on how to build and plumb more efficiently (for way less money!) for next time around. So many parts that I imagined would be easy to find were not, and some didn't even exist. I've kept all my receipts and will update with the way-over-budget total when I'm done and have returned all of the incorrect parts that I bought. :) I will also add a more reasonable hindsight budget for next time!
The system is a roughly 100 gallon grow bed and roughly 100 gallon fish tank. Both are built with 3/4" plywood and reinforced in the corners (top and bottom) and lined with EPDM pond liner. I will probably also add some reinforcement going over the top of the grow bed to keep it from bowing over time. It's all on an old shelving unit my carpenter neighbor built and gave to me when he moved. Very sturdy but added more casters in the middle (you can't see them in this picture) and some extra 2 by 4 bracing just to be extra sure. The system can obviously not be moved around when full, but the idea is to empty the tank (even temporarily) between seasons so it can be in afternoon shade in summer and full sun in winter. When it's in place I have cinder blocks underneath it for extra insurance.
The most simple explanation is that a pump pumps about 40% of the water out of the fish tank to fill the grow bed, when the water reaches a certain level a siphon kicks in and floods the water back into the fish tank. I went with an Affnan siphon for those of you that are interested in those sorts of details. I'll do a more complete post about siphoning in the future.
Looking down into the growbed-- need a little more substrate to fill it up.
The rocks on a plate are a temporary weight to hold down that pipe. The pipe has lots of holes drilled in it to act as a strainer to keep the rocks from entering the siphon, which is inside that pipe. The pipe to the left of that is an overflow pipe (I need to make it shorter). It's simply a back-up in case the siphon ever fails, it will keep the grow bed from overflowing. The other pipe is hooked up to a 35 watt(!) Laguna Max Flo 600gph pump, and carries the water from the tank to the grow bed. I will put some aquarium filter underneath where it comes out to keep too many solids from entering the grow bed. It might be hard to see but the water pipe is zip tied in place in three different spots to be sure it stays in place.
Still a little more fine tuning, but as it is now, everything works how it should. I plan on beginning to cycle the tank tomorrow, it should be ready for the tilapia in about 3 weeks.