Making an inside-wall home aquarium?

My advice to you would be rather than cutting a hole expose the block work after cutting away the plaster then take the block work down starting from the ceiling and working your way down to your required position,make up a template for you tank insert it and then build your block work back up and install your lintel at the required height and build back up to the ceiling again and re plaster the wall then then no risk of anything collapsing and using the template will give you some idea of what it will look like once you are done ok,good luck.