For proper functioning, UV devices usually have recommended flow rates - to function optimally, the water through the device should be "in contact" with the UV light for a minimal amount of time. If the water flows through too fast, the device won't be as effective. Slower should be OK, although too slow doesn't move enough of the tank vvolume through. As long as your powerhead can provide the recommended flow rate, it should work OK. Don't forget that the UV sleeve should be kept clean to allow the best UV penetration. perhaps your undergravel filter will cause the sleeve to become dirty quicker.