^ this may be likely. The tank glass may be fine, the stand itself may be level, BUT the stand surface may be warped, especially around the edges.
Not sure how the tank stand was designed, but you could test by cutting another plywood for the top and place the tank on. At least to get an idea if it's tank or the stand. I used 3/4" MDO plywood (don't mistake for MDF ;-)) as I found it to be more flat and true. They make outdoor signs with this stuff.
Either way, I would take it somewhere else to do the fill test in case it does leak since you've not tested yet.
Not sure how the tank stand was designed, but you could test by cutting another plywood for the top and place the tank on. At least to get an idea if it's tank or the stand. I used 3/4" MDO plywood (don't mistake for MDF ;-)) as I found it to be more flat and true. They make outdoor signs with this stuff.
Either way, I would take it somewhere else to do the fill test in case it does leak since you've not tested yet.