If the silicone was used to patch the crack in the trim, there is no issue with the tank. The trim is not necessary for the integrity of the tank. I would use that tank without a second thought.
The stand is either not level, which is the cause of the one corner being lower. The bad thing about this is the tank is trying to twist. The stand needs to be levelled, and the top made flat. If, in fact, the corner that is overhanging is warped down, you can rectify that by adding a piece of 3/4" plywood to the top. Additionally, a piece of styro about 1/2" to 3/4" thick can be placed between the tank and top to absorb any minor discrepencies in flatness.
One thing that can make a tank lookout of level is if the trim pieces are not on evenly. Measure the distance between the top and bottom trim in the four corners, to see if they are the same. It looks to me like the stand is out of level and you can rectify that with some shims. The tank itself doesn't look like an issue to me. I would be glad to have it.