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That should be good. Over is better than under. Just position them so the sand doesn't blow all over. I have 3 MJ1200's in mine, plus the overflow pump. The MJ1200's are positioned: 1 in the center, and the other 2 in the back corners facing towards the middle of the tank. Try to keep them close to the surface for water agitation for better gas exchange.
In addition remember you dont want a jet stream of water instead you want good random water movements. TO do this you can position powerheads toward eachother so the water interacts with the other powerhead's current.
I got myself a Tunze stream kit to replace 6X maxi 12s and I am really happy with it. It pushes about 3800 gallons an hour in my 105G tank, but the corals are eating it up and the included wave controller makes a very realistic wave action. They are kind of pricey and very cheaply made, but they work like a champ.
I would say yes, you need the power heads to circculate the water to keep O2 lvls up. You probably don't need as much as the coral keepers but you deffinately need the water to be moving otherwise it will get stagnant.
Your going to need something moving the water. Certin algaes will grow much better in stagnant or semistagnant water so often just some water movement of 7X to 1 will keep some forms of algae from going out of control. In addition your SW fish have come from a habitat that has fast moving currents and on a health stand point putting them in a tank with nearly no current will not be good for them. Some fish, namly tangs and other big fish, will need these currents else they can sufficate or get them selves into trouble as they try to make their own currents.
For your size tank though a tang is way to big. I just used them as an example. You should shoot for 300-450GPH of overall water flow in the tank itself.
You need circulation. Powerheads are the easiest way to achieve that amount necessary. I have an oceanic 32 hex and I currently am running a maxijet 750 and a fluval 104 (which means I barely meeting the min 10x gallon requirement for a saltwater tank). What are you using for circulation? I plan on adding another powerhead to increase circulation and surface agitation on my tank.
Actually I was thinking I needed one anyway, I am concerned later on it may not be enough oxygen, so I will get one.
160 GPH is what I am going for (I am planning on small fish and dont want them to have harder time swimming, this would also keep the water moving pretty good too)
I added a Tunze t24 kit as well. You can set the flow at your desire and change it up in a ton of different ways not to mention I can turn he tank over 40 times per hour if need be. I have it set at around 25 times per hour right now not to mention my mag 18 return pump. However you only need to go this extreme if you plan on an sps tank. Flow is needed for a healthy aquarium and solved my ph crashing problems at the same time.