I'd be a little wary...
The way it looks in the photos, All of the weight of the tank is going to be supported by 8 screws. It would be better if you had the uprights stacked between the top and bottom rims, or if you put a sheet of plywood across the entire top surface of the tank.
As to the shelf... You might be able to work with that, as 6 gallons isn't that much, but I'd feel a lot better if it was directly supported. You could easily make some 2x4 legs long enough to sit under the shelf and support it, and then just screw/glue them onto the sides of the existing leg.
I feel that screws should only be used to clamp pieces together for gluing, or to help make assembly easier, they really shouldn't be load-bearing.
A good, solid stand design should have direct aquarium-wood-wood-floor stacking, and you should be able to hold the stand together with masking tape. If masking tape won't hold the structural elements together, it's putting too much stress on the fasteners.
It looks like you've got pretty good fit, and everything measured out well, so if you made a couple minor adjustments/additions, you could easily have a bomb-proof stand.