Stevepmd, i do run RDWC if i didn`t tell ya and still by the tenth day i usually change the resevoir. What i meant topping off is during the 10 day period i just check the ph to make sure its good and fill the res if necesary. By having a pump to help keep it recirulating the nutrients helps so you can let it go longer than just DWC where i was changing the res every 7th day compared to 10 days or more in RDWC. Check this You Tube site out PAhydroponics. Guy named Gary has some very knowledgeable videos and sells pre-cut parts to make your own DWC or RDWC for low cost prices.