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https://www.swelluk.com:443/swell-maxi-air-pump-replacement-diaphragms/ 
Swell Maxi Air Pump Replacement Diaphragms
Replacement Diaphragm for the Swell maxi Air Pumps for models 600 - 3600
Products
Product Information
Diaphragms are a moving part within the Pond Maxi Air Pump and over time will stretch and can split, reducing the output of the air pump. They are made from a silicone rubber for excellent reliability and life expectancy but will eventually need replacing.
Diaphragms are a moving part of the product and are not covered under warranty.