I use a Jett fueler but a buddy of mine has used a large syringe.
You can go here http://www.jettengineering.com/ and get a good idea of how all of this works.
Since the bag expands to the size of the tank you can feel the fuel stop going into the tank. If you use a pump you run the risk of bursting the bag and damaging the tank.

There seems to be a misunderstanding of bladder tanks. Pylon racers use the bladder tank with just two lines. Fill/Feed and pressure. This configuration requires a syringe to completely empty the bladder and then fill it. The Jett tanks are two line and meant exclusively for Pylon Racing.

The Tettra tanks can be set up in a two or three line configuration. The Tettra instructions show only the three line configuration. Fill/Feed, Pressure and overflow. They fill, feed and empty just the same as any other three line tank. Whether the pump used to fill is hand or electric there is no more danger of bursting one of these than there is of any other tank. Fuel will pour out of the overflow when the tank is full. The overflow has to be plugged when the tank is full. Dare I say it again? Just like any other three line tank!

Ed S

The picture shows a selection of Jett and Tettra tanks.

Thanks, that's what I needed to know (third line for overflow).

Still, it doesn't sound "like any other tank", even for a three line tank. A regular three line setup gets filled via the fill line and overflow comes out the muffler line when it's full. The carb line stays connected all the time and the fill line gets plugged for flight. With the Tettra, it sounds like you fill through the carb line and watch for overflow out the overflow line, leaving the muffler line connected all the time and plugging the overflow line for flight.

Hi!
Yeah!
You are right...but isn't that about the same way as any other tank!?

Correct because the muffler line is on the outside of the bag inside the tank. Your muffler pressure never comes in contact with the fuel.

"About the same" doesn't cut it when I'm trying to figure out how it works. Ed straightened me out with his last explanation, though.