You should run more then just one airstone. You could burn up your air pump. You want to be running enough tube to equal the size of the pump nozzle. 3 minimum is my estimation. It still gets warm but not as much as when I was using fewer stones.
This is what I want to get sooner rather then later. DIY PVC Airlift Compost Tea Brewer - YouTube
How to make an air lift tea brewer for GIANT YIELDS! Designed by MrTimjwilson - YouTube
Looks simple enough right?