Power Heads and Air

[Skydiver]

Does any one know how inportant it is to use the air "injection" valve to introduce air into the water stream that the power head shoots out for areation? Should you add air or is the surface agitation it creates enough to replace O2 in the water? I have a 55gal tank and 2x 550  Penguin power heads.

 

[dattack]

If you have turbulence created on the water surface, this is be just as efficient in terms of getting oxygenation for your tank.  That's why if you have a power filter only, the turbulence itself will create the gas exchange.
For powerheads if you have it pointing to the surface where it create waves, then this will provide adequate oxygenation for the tank.  If you do not have surface agitation, then you will probably need to use the venturi on the powerhead to ensure oxygenation.
Either way will work.