There are different approaches that can be taken here, and there are pros and cons to each.
I would recommend waiting until the concrete has reached design strength and then backfill and water test. I would recommend backfilling first as the backfill will support the walls from flexing and cracking. Cracks that do form or are currently existing may leak for several days. This is normal as Xypex takes time and water in order to generate a crystal structure across a crack.
The tank can be tested prior to backfilling but there would be expected to be more cracking on filling, as the concrete will not be supported, and it will still take several days for the cracks to heal. You will therefore be waiting quite a while to backfill.
Any cracks that do not heal will need to be routed and filled and these will certainly be easier to identify without the backfill. If the tank is full the use of dye can help identify leaking cracks and then the tank can be emptied and repaired.
Posted on 17 Apr 2013