Crop salt like I said before is gna be ur best bet and most bang for ur buck more or less and have full control I'd also highly recommend u run silica of some type either like power si or u could do organic silica even too in dry forum but most def need to run it and to clean and sterilize u should get bio safe system green clean acid clean and to sterilize either oxine and citric acid or h2o2 etc can work but you must fully clean and sterilize after every crop and u need to have access to all the plumbing ..not sure if ur building ur rdwc but u need to do it like how I did mine using tees for the ends of the drains and using a test plug to cap it off and u have easy access to all the plumbing that way vs a elbow which has zero access after the bend....and u don't need to do water changes unless u have a problem!!!!!