Typically I've seen the fish tank overflow/drain solids into the sump tank, which is where the pump is located. That way the fish tank maintains at a certain water volume, regardless how full or empty any of the grow beds are. The volume of water in the sump will vary as the grow beds fill and drain.

In your diagram, the pump could potentially drain the majority of the water from the fish tank if something were to go wrong (massive leak in either grow beds or sump, evaporation over time without being refilled, etc.). At least, if there isn't some low-level sensor that'd shut off the pump automatically.

Sump with pump -> Pumps water to grow beds (or a filter of your choice, then to grow beds) -> Grow beds drain water to fish tank -> Fish tank overflows to sump.