I only have a 5 gal bucket res, so I just use this little $8 pump -http://
www.amazon.com/Uniclife-Submersible-Powerhead-Hydroponic-Certification/dp/B00ZW6OHHY/ref=sr_1_6?ie=UTF8&qid=1456512413&sr=8-6&keywords=water+pump
Mine is a 80GPH(gallon per hour) and they make a 400GPH for only $15 so you could just get that one.
But exactly as kmog said, once I'm in flower my plant drinks the whole res weekly so I just pump the small amount of water out of the bottom of the res and pump the new stuff in. When I'm in veg and my plant & roots are small enough for me to lift and set in another bucket i actually take the res bucket to the tub and empty it and spray it out with the shower head so that I don't have residue build up(if you wipe the sides of res with paper towels you'll see what i mean by build up) even though once i get to flower i can only pump, but i figure any bit helps. Root rots the mother fuckin devil so anything to not get it I say.
While on the topic one thing I can suggest is to at least make sure once the grow is done you give your res a really really good cleaning. If you have any hoses for water level make sure you clean those fuckers out, I did two grows and then on my third grow I got root rot and the only thing i can contribute it to is that there was gunky crap build up in that water level tube that I found when cleaning the shit outta my res. Keep in mind little parts of the roots brake off every now and then and you'll see them floatin around at the bottom, those will eventually sit in the tube and start forming gunky crap once they all collect into a bunch.
