You want something that is completely contained INSIDE the 7 inch thick wall, right?
You could have a rectangular tank 30 x 36 x 7in custom built of acrylic. Why rectangular? Because of the surface area issue that jsoong brought up. This 32 gal
tank would have 30 x 7 = 210 sq inches of surface area - bacause of this, you would only be able to stock it like a 15 gal
. Why 36 inches high? The extra height would hide the waterline, filter output, and heater above the circular opening on both sides. Other than the expense, this tank wouldn't be difficult to build.
You would certainly have to add more vertical studs and double up the horizontal base plate and header in the space below that tank to support the weight (about 300 pounds). I would also double uo the vertical studs on either side of the tank. This would involve tearing up and replacing the wallboard on the walkway side.
You would almost certainly have to use a cannister filter because I'm sure you don't want an ugly HOB
sticking out of the wall on either side. The cannister filter could sit on a shelf built in the space adjacent to the tank - behind a removable panel. You would have to find a cannister filter that would fit within the 7 inch wall space and drill a hole through the wall stud for the hoses to pass through. I notice an electrical outlet and wall switches on the wall - handy because you could tap them to power the filter and heater - might require more wallboard replacement and drilling through studs for the wiring
You would need a large vented access panel above the tank. Feeding and cleaning would likely require a stepladder and cleaning would probably be a major pain.
This would be a visually stunning one-of-a-kind setup, but in the end, you would have a difficult-to-maintain 15 gal
tank for the price of a 75 gal
with a lot of carpentry work thrown in too. If it were my wall, I'd get a circular piece of plexiglas with a nice stained glass applique decal for the opening....and spend all that money on a 75 gal
setup to be placed elsewhere in the house.