I usually change my water when I switch to flowering cycle and then flush my tank about 2 or 3 days before harvest. Pureblend brand is what I usually use...relatively cheap and it works
I'm going to assume (maybe incorrectly) that you are either new to growing or at least new to hydro...if that's the case, making things easier should be a priority... Look into the Lucas Formula...http://forum.grasscity.com/hydroponic-growing/615009-lucas-formula.html Many hydro growers have used this successfully for years...