Both growbed done, complete with aeration pump and almost 12 inches of media. Another day of back breaking work and now I will just need to cycle the growbed for a few days "dry".
Only left are to tidy the electrical points, get a timer and few starter fish. This photo shows water at its highest point in the tank, with both growbed empty. The difference between lowest and highest is about three inches with these two growbed configuration.
I already ordered another two rack since I'm confident that this fish tank will able to accommodate four thirty gallon growbed as I normally use above.
I will try hydroton on the new beds and see how its goes.