i've just been reading upon hydro feeding and from what i can gather you can do a couple of ways, firstly just top the res up with plain water to your res and dump out and refill once a week or you caan arse on daily taking ph readings and ec and work out how much food the plans taking on board and add nutes plus water.
the second being a shit load more precise but you'll constantly be riding the edge of whats happening balance wise. the first means you will constantly underfeed your plants. or there's the half way house of mixing a low amount of nutes but not just plain water.
I'm going to get a waterfarm and go with plan A first. then see if i can go onto the third option.