池塘 is a natural or garden pond. For swimming pool use 游泳池 (yóuyǒngchí). Both 池 and 塘 individually mean 'pond,' so 池塘 is a synonym compound that reinforces the meaning.
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Both characters have the water radical (氵and 土 as earth-bank): 池 is the water, 塘 is the embankment — together, a contained body of water.
