Reinforcing furniture for aquarium
It may be strong enough to put that aquarium up there but there are a raft of factors that would seem, when taken all together, to discourage you from pursuing this idea. 


Aquariums require regular servicing and having it up so high will be a total pain in the tush when it comes to cleaning and adding water and so forth.
Cleaning aquariums is usually a wet and messy job. The possibility of spilling water down over electronics or water damage to the "decent" looking furniture is a high risk.
The backer board in that shelving unit may very well be made of a thin cardboard material that has a nice wood grain printed surface. Such material would not stand up to providing side to side stiffening of the unit with all that weight on top even if you laid flat 2x12's across the top of the unit.
The upper shelving unit is not built into the lower unit as an integral structure. This leads to concern the unit could buckle out the middle under  possible earth quake conditions even if the top of the upper unit were to be secured to the wall. 
If you have kids in the house, or ever plan, to be aware that they can and will attempt to climb up on shelves. An aquarium up there is bound to capture attention or curiosity. It would be extremely unfortunate if a child were attempting to climb and the very top heavy shelf and aquarium were to collapse, fall or be pulled down with said child underneath.
From a pure aesthetics standpoint even the empty aquarium looks very top heavy up there to the point of being almost ugly in this observer's opinion.