Not sure what you mean by "upper extension". Are you talking about the tank that is full in the video and running?
Your nano is going to weigh in probably just under 200 pounds. Sit up there and see how sturdy it is.
Keep in mind too that plywood doesn't have great long term deflection characteristics. Meaning, you can put a load onto plywood and it will not deflect much. Overtime though, the fibers and layers of the plywood start to flex which ultimately means that it starts to deflect more. Water in plywood greatly accelerates this process.
You span is so short, I think you will be fine, long term as well.