Are you done building the stand? If so I would certainly consider putting some horizontal cross bracing on at least 3 sides. Stands constructed like this have a tendency to lean to one side or the other. The plywood at the top does have much structure to hold the wood in place and prevent it from tipping one side or the other.
I’d recommend boxing the wood in at the top with a frame or skinning the outside with some plywood to hold everything in place.
It looks like it’s pushed to the left. You can see in the one photo the right has a few inches of plywood on it where the tank can be pushed over.
Good catch though because you don’t want the corner unsupported!! That would be disastrous.