I use a Whale submersible pump, designed as a pressure pump for fresh water for caravans, etc. Says max run time 15 mins at 12V, but I use 12V to purge the air at the start of a session then switch to 5V and it runs silently and happily for hours on end. Sits in a bucket of water and connects via cheap pvc tubing. Cost about a tenner, about 4 years ago, and still running fine.