"Read the wake" to make sure you're weighted evenly.
Check out this thread for wake shape and 'reading the wake'
http://www.wakeworld.com/forum/showthread.php?t=804667
Once you know how to read the wake (look for a clean wake with no white wash):
1. Fill hard tank full
2. Fill the bag to where you think its close to the same weight as the hard tank.
3. Read the wake
3a. If the wake is washy on port side:
Add more weight to the bag on the port side or
Drain weight from the hard tank on starboard side until both wakes become clean
3b. If the wake is washy on starboard side:
Drain weight from the bag on the port side until both wakes become clean
Note that speed also affects the wake shape and where it washes out. I'm assuming you have already dialled in your speed (and line lengths) with the hard tanks in the boat.