Every pond has it's unique pumping needs.
We find that submersible pumps tend to wear out after 1 year from debris getting inside and damaging the impeller. The external pump's pre-filtering is on the outside of the pond and people tend to empty the basket more often because it is more convenient plus the pump itself is not sitting in the sludge on the bottom of the pond. The external pump tends to last MUCH longer before needing any repairs or replacement. The CAL Pump carries a 1-yr. When a part goes out on an external pump it can be replaced quite easily.