well for hydro, roe and distilled is pretty much the same thing, ur looking at ppm differnce of 0 to like 30, its just which sytems is easier and less money. The RO systems ive never herd of forming bacteria, u just need to replace the filters when needed. this is all drinking water if ur getting bacteria in ur RO water its comming from somewhere else. If your concerned with the water ur using im assuming your using some nutes at some point in ur grow, so leeching shouldnt happen if u feed them some food in the water. the point of this isnt to feed your plants pure water, its to take out all the crap and put in only what u need, u should never feed them strait up 0ppm unless your flushing. personally i use my well water which sits at a very high ppm around 500, i used 1/2 stenght nutes, no filters, no problems, so its not neccessarily worth all it is. cause i dont use anything and i get 100% resluts. My water is filled with goodies i dont even know are in there, lot of calciuim, mg, can be a problem with some strains though! Fliters i would say are more for the experienced grower who are using hard strains that required very little fluctuation in water ppm.