will a main pipe bring the solution to fill all fish tanks equal so that all have the same flow rate?
In order to get the same flow rate, it would need to be under pressure, and would have to have valves that you can adjust. It would be easiest with using a manifold and separate lines. I think you might be able to do it using gravity, but it would take a lot of trial and effort. You would still need valves to increase or reduce the flow though.

I would also like to suggest an over flow that bypasses the fish tanks in case the flows to the fish tanks can not keep up . Just a thought.

Jason