Definitely 20 amp circuits, most use 20amp for any outlet regardless of it's function over 15amps which is very often left for lighting circuits (not tank lighting

).
As for how many, depends what you're going to use. Think mostly of big ticket items mostly.
- How many gallons is the tank? Heating is a foregone conclusion, are you going to have a chiller?
- What kind of lights are you putting over your tank? a few 400w metal halides?
- What kind of return pumps are you using? If they mention some fraction of horsepower on them, they're going to be energy hogs. In tank pumps usually are negligible.
Those three are usually your biggest energy worries. If it is a small tank, maybe 100g or less I might be ok with a single 20amp circuit, maybe 2 just for extra. Do a quick energy audit of your equipment, figure approximately 2000 watts per 20amp circuit but if you get close to 2000 you might want to just add another to take into account inrush current from certain devices like lighting or any mechanical pump that turns on/off like a chiller.
Oh and raising the height of the outlet is fine. I might put them as high as you can get away if they're anywhere near where water might splash (like a sump)