What happens is that I change the valve in the 1st picture and the blue tank immediately starts to go lower, the pool fills up and flows over the sides into the trough, but then the blue tank starts to fill up again from the water inlet, after about 5 mins, a little bit of water starts to come out of the exit pipe and keeps coming out.
Edit: also, when someone dives into the pool and sloshes water off the negative edge, the trough catches everything but then a whole load of water comes out of the blue tank's escape pipe.
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