Obviously your in-wall tank needs to plug in somewhere to run the lights, filter and heater. First, any outlets around the tank should be GFI (ground fault interrupt). This is the same type that you have in your washroom. It will prevent any shock hazards from the water in the tank.
This outlet should be inside your wall opening so you do not see it when you are looking at your tank. Typically it will be hidden under some trim out of view. If your opening is tight do not place it in the top center. It might get in the way of the light (the top corner is preferred).
3 – 4 devices will run off of this outlet so either a double outlet or power-bar will be required in the end. Any wire slack can be stapled out of the way. Do not worry about seeing the wires off to the side, when you look into the aquarium the ends inside the wall will reflect the light inside the tank like a mirror.