I like the hydor powerheads. Easy to clean, inexpensive, and do a really nice job keeping the water moving. As far as how many...to hard to say cause the size of the powerheads available. Also how much flow do you want in your tank? Do you have corals or livestock that prefer calmer water? Anyways I would suggest at least two no matter the size. One to create a nice water flow throughout the tank and the other aimed towards the water surface giving it a little turbulance. Just my two cents.