How heavy is the top tank? Less than 50lbs? You could go to a welding shop or metal working shop and get to L shap pieces of metal and bolt them into the wall.
If you don't want the bottom part of the L visable, and you aren't afraid of drywall work, you could open up the wall and attache the brackets inside the wall with just the top part of the L sticking out.....
You could go about supporting the tank in a completely different manner, and suspend or hang it from the ceiling.....with some nice slender steel cables or something. Maybe all-thread rod.....
That's all I got at the moment.