If your fuel tank is expanding/contracting during fueling/defueling it's because your vent is too restrictive for what your pump system is delivering. There are several things you can do to prevent or minimize this:
- Go with larger diameter tubes and lines on your tank vent and between tanks.
- Put a restrictor in line on your pump system or go with smaller diameter lines to reduce its flow.
- Reduce the voltage on your pump system.

Since I have one main tank plus a UAT I fill directly into the main tank.

Joe