First confirm wether your pump works on AC orr DC. Coz you may use a 9 volt transformor and proportinally the pump will be having lesser pressure when compared connecting with regular power supply of 12 volts. Naturally 12 volt supply will work better with the same. Also have a look at the current requirements and select a power supply which can cater the same. CheersMarsbar4 said:I have a 12v output pump in my aqarium and have :?:a 9v output transformer, can I use it or what other transformer would be better?