I have a nissan altima 2006 ive replaced the radiator, the fan, and the thermostat, and its still overheating when i let it idle or drive it. What else could it be?

Water pump.

I thought that but its retaining all the coolant it doesnt leak so i thought it couldnt be that my last car when it needed the pump i had to constantly put water in it. But it can still be the water pump even if it dont need fluids added?

Yes.

http://www.underhoodservice.com/tech-feature-cooling-system-and-water-pump-service-for-the-nissan-3-5l-v6/

Its a 2.5 is there a big difference

Oh. I stand corrected. I was looking around for information and never was a mention of a 2.5 Sorry 'bout that. But it's possible that the shaft has sheared between driver pulley and impellers.. or woodruf or dowel pin- whatever keeps the pulley attached to pump shaft and pulley is spinning fine, and all seems well except the impellers are not turning... hence no leak but no circulation either