You need to set the return pump to pump less GPH than the overflow on the tank is rated for. this way when water is pumped in the DT it will overflow just as much back into the sump.
You do this by adding a valve on the return side pipe going up to the DT from the sump.
Just a note, I've had catalina equipment before and haven't had good luck with either the pump or the heater I had. may want to budget an upgrade for the pump itself.