I've been thinking a lot about how to design a system in a 4m x 3M Keder house. I am coming around to the idea from Murray Hallam, that perhaps a sump is a good idea, mainly from the point of view of relative levels. The fish tank could sit on the ground, and the grow beds could be fairly low (which I would prefer, for access to climbing plants) especially if the sump was sunk into the ground. It could be completely insulated to help maintain temperature, and the growbeds could be insulated too. Another plus is that much of the pipework could be run underground, which, especially if lagged, would also help to retain temperature.
I needed to find out how big the sump needs to be, so I asked Charlie Price about the medium/water ratio in a grow bed. (He said it's about 2/3 to 1/3) I have worked out that if I have five bath-tub growbeds, I would need a sump with a capacity of around 450 litres.
I have asked Murray Hallam if I would be able to use the sump as a breeder tank - which could solve the fish stock problems. I read that you can have a breeder tank for Tilapia, and if you have a male of one type (specified) and I think five females of a different type (specified), the offspring are 98% male, and it occurred to me that these could live in the sump!
Now I have to find a source for a 500 litre tank of the right shape, preferably without buying two IBCs!