starting a new tank

[RuGlu6]

If there are two pumps one pumping from tank one in to tank two and the other pump is pumping back, it is a recipe for disaster.
Two pumps are never same even if they are "identical" there will be variation in an amount of water they will pump.
The only way to safely connect two vessels is an overflow with single circulation pump and the way to do it is to place one tank slightly above the other.