You added more flow thru the tank with the new pump. If it won't level off until the tank overflows then the wier( the thing that looks like a comb that leads to the back of the tank) can't handle the added flow.
Easiest thing to do is slow the pump. Can you get your hands on a ball valve? It will help you tune the pump to match your wier. You will need to know the ID of your hose.
Specifications: Material of construction: Gray PVC Long Lever Handle 1/2" Barb x 1/2" Barb Buna-N o-ring Max Pressure: 125 PSI Max Temperature: 140° F Ball Through Hole Diameter: 0.437 in
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