HI, I was just in OAHU last week at the Marine Corps Base, Kaneohe Bay. Before I left, I went to an aquaponic farm. Check out Olomana Gardens in Waimanalo, HI. Glenn Matinez has developed an air-pump that uses a low power consumption air compressor.
With regards to the mixing trays, try doubling up the mixing trays. The top tray is drilled with a lot of holes, like a pasta strainer. The lower tray has a drain fitting attached. small spacers are placed between the two trays. The drains from your three grow beds can be connected together and one siphon can drain them all... This is an idea from Glenn as well
You can tour the farm in person for 25 dollars. If you cant make it over, there are tons of videos. I highly recommend stopping by. I found it to be worth it.
As far as a dc pump is concerned, you can look into a small bilge pump from a marine center. Good luck