I use a 2.5-3 inch base of RINSED lava rock In the bottom of my buckets...It creates a barrier that most of the roots will COIL up on when they get there...anyone who uses a 'rock basin' has seen how roots coil up ON TOP of it....It keeps the root mass from going right to the bottom and sitting in 2inches of water...So the lava rock base is where my water reservoirs are...the PVC tubes are 45° cut and go to the bottom of the BUCKET....If I have a need to FLUSH the res, I do it by watering down those tubes...TOP WATER FLUSHING is hard on the roots and unnecessary...There is no build up in the medium....It goes straight to the lava rock basin, and is 'CLEANING' those out nice, without having to TOP WATER flush.....NOT SURE how many ppl have actually TESTED their internal res #'s...THEY CAN easily drift, and this is where your plants are getting water/nute Mix from....AND they are getting all of it from there btwn waterings....I have a 'rubber plug' in each bucket on the sides, AT THE BASE, of each one....It can be pulled, or loosened, to get a water sample from the internal res....Its best to check it, THE DAY AFTER a watering to see how your numbers behave....TDS can go UP or down depending on how much you feed them And the pH can easily drift along with it....My TDS and pH like to drift up, and can be boarder line 'out of spec' by the 3-4 watering, IF YOU DONT UNDERSTAND how they are feeding......since they are growing their nute needs change and even though I've done it a while, it will drift...taking water samples has helped me better understand HOW they are feeding which keeps MY water/nute program tuned better...Just checking Runoff is NOT an accurate way since that sample is very 'CUT' With what your watering with at the moment....Ppl got to understand that EVEN THOUGH your letting your RES water be depleted btwn watering, TDS and pH drift STILL occurs....EVERYTHING in buckets base can have residual nutes and at an undesirable pH....IF MY NUMBERS DRIFT TOO HIGH (Or possibly too low in someone elses case), I can immediately flush down those PVC tubes to a good runoff....I like keeping my #'s exactly where they should be.....TOOLS for better hempy growing....