the easiest way is to draw it out on paper. where are you going to put everything. put a few under your stand, on each side (skimmer, and reactors on one side, return pumps and closed loop on the other. (if you are going to use closed loop now or in the future.) put a seperate run in the celing just for lights. you will want them on their own circut. then, you will want a few around the room for misc items in the future. (you think you will be done buying and modifying, but it never stops) think of where a chiller will be now or later, you will want a fan for circulation. maybe a fan on the lights and on the sump. outlets for saltwater mixing, and maybe a pump for your RO. these are just a few. i wish i would have taken the time to do this. extension cords suck and look nasty. not to mention they get in the way.
i hope this helps.
not to mention, an extra outlet can choose not be used, but no outlet can not be used.
if it is unused, you can get those splash proof guards.
