yup 30 sounds about right,buy a 5 gallon bucket and mix the exact same amount of nutrient solution in each, now for your ppm and ph flux in each container,this will even out and they should all be the same after a couple of hours.its auctually the whole reason for using a recirculating system,to combat your left over water, first,pump out as much as u can with your water pump,member i sed it was a good idea to get xxx amount of feet for the half inch to do this,second.raise your 2 main growing chambers up off the ground using milk crates "anything will work".this will drain all the remaining water from the grow chambers to the rez via a gravity feed.third,this is were the 2 on/off valve on the rez come into play once the grow chambers are empty and the rez is now holding the remaining water kick your pump back on.dump what little bit of water u have left into the 5 gallon bucket your going to buy to mix your nutes with, take it 2 the sink.clean it out and add your new solution.this should be done every 7 days.