If you were to use a pump, lets say for example one that pumps 200 gallons per hour from the main tank to the sump, then you will also have to have a pump that returns it to the main right?
So lets say you use another pump that is 200 gallons per hour from the sump to the main. If both somehow work perfect then your water would circulate fine and evenly.
But what two pumps are going to be 200 gph exactly?
Even if they did work perfect, over time one or both will start to get calcium buildup on them, and I just can't believe that they will both be coated exactly alike. As they get coated with calcium or even algae then this will slow the preformance and you will not have and even flow keeping the water level in your sump perfect.
The sump pump could get faster causing your main to overflow and your sump to depleat to the point of running your pump dry and burning up. Or visa versa dependin on how you placed your main tank pump to the sump.
Get it?
Thomas