pumps are great product for a small and/or medium ponds
(1000 gallons or less).
When choosing which submersible pump to get for your
pond, think in terms of the gallons of your pond. A pond of 400 gallons
should have at least a 400 gph pump -- you want your submersible pump
to circulate all the water in the pond at least once an hour.
Many find that submersible pumps are very economical
and also practical. They can be placed directly into the pond (as
opposed to external pumps which, as their name implies, are out-of-pond
New models of submersible pumps are magnetic driven.
Watch out for older models which required coolant (coolant is fine
-- unless it leaks into your pond!)