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When you say air pump, do you mean you will be hooking up an air stone or bubble wall to the pump?
It doesn't hurt anything. As long as the filter is providing adequate water agitation on the top of the water, then adding any kind of air pump is essentially useless to the fish.
If you had plants in the tank I would say not to use an air pump as that would drive off CO2. But if you have a non-planted tank, go ahead and put one in.
If you have a HOB filter, like most folks do these days, you probably notice that the water dropping into the tank makes bubbles. This puts oxygen into the water. I have two ten gallons set up this way. They have no air pump and the fish are all fine.
Now, if you have a canister filter that releases the water down inside that tank (so that there's no surface agitation), you may want to put an air pump into your tank.
Also, you could put an air stone/bubble wall in if you think bubbles are attractive. I am thinking about putting a bubble wall on the back side of one of my tanks. Not because the fish need them, but because I like the way they look.
I just bought this really neat flexible bubble bar. totally awesome... I recommend it, if you are just looking for the bubble effect. No. desn't really do anything for the fish.... well, a few of mine like playing in the bubbles... but look great. Check it out here:http://www.marinedepot.com/a_ap_va_as.asp