Redoing the passive auto changeover valve

Pfh offered to help us prepared the house for departure, and one of the things which needed to be fixed was the connection between the watering system solenoids and the passive auto-switchover system. The old connection was a piece of polypipe which was beginning to look sun damaged. I also wanted to bring tank water into the laundry to run the washing machine on. May as well do both in copper at the same time:

Pfh designed the layout and cut the pieces, which I (and mental) think has a certain "steampunk charm". I soldered the pipes.

Tif has offered to check over the garden from time to time in return for various things, one of which was greenhouse space. We discussed my flood and drain cuttings system and how I'd been stymied by a lack of suitable growbeds that would fit in the space. She suggested I look at storage boxes, and sure enough, we popped into B on the way home one day and found the perfect box - fits both of my tray systems snugly. I tried using a flange (see previous posts) and a rubber washer to make the outlet, but It was too hard to make a sufficient force to hold the seal tightly in place. Whilst getting a few bits for the copper plumbing work pfh found a nifty through thread piece that gave us a sealing through hole with barbed 19mm tee on the outside.

So the whole system is almost done, the only issue to resolve is the overflow arrangement. The pump turns on and water bubbles out the hole, then the pump turns off, and the water drains back out the hole. One problem is that the water forms a thin layer on the bottom even when the rest of the system has drained back to the fish tank. Will think about solutions to this.