My first suggestion would be to try soil out.
If you don't want to do that, get an Reverse osmosis water system and add cal-mag to that up to 100-150ppm maybe some veg nutrients up to 175ppm total. Make sure your PH is right on 5.8 constantly. If not that, then use regular tap water PH'd to 5.8 (Really difficult to keep the PH at 5.8 with some peoples tap water... like mine)
Also, dump what is in your res/bucket and CLEAN THEM LIKE CRAZY!!!!
The dark leaves are usually a sign of to much Nitrogen, so clean everything and put a low ppm solution or PH'd tap water in there.
I just did a DWC run, and since mine was not recirculating and i used a screen, it was an uphill battle from day 1 of flowering. If everything is perfect DWC kicks ass, but for me its just too much work.
Also, as the previous poster said, check your airstone to make sure its OK.
Hope I helped in some way!