Growonix or even Hyrdologic I think are still overpriced. They work well but there are many options that work better and are cheaper. Imo, anything from the store is overpriced too. I use a 5 stage RO/DI system for 0ppm water from purelyH20.com. They custom make it and use the best membranes available (Filmtec). Bulkreefsupply.com, thefilterguys.biz, and purelyh20.com are all experts in water purification and are SO much cheaper. Not to mention the ones you get from these guys last for up to 25,000 gallons vs only 1000 that you get from the lowes and home depot junk. Less filters equals less money. You do the math. Check my sig, I have a pic of my Ro/di unit I bought from these guys. I LOVE IT SO MUCH!!
Check this one out if you want
http://www.purelyh2o.com/index.php/h2o-test-meters.html?page=shop.product_details&flypage=flypage.tpl&product_id=23&category_id=79
Also watch this video on how to select an RO system. (yes, he says its for tanks but we use ro water also)
[video=youtube;1QKJbO4UBJw]http://www.youtube.com/watch?v=1QKJbO4UBJw&list=FLS7_AXHPeHw2X4CxJm64y7w& index=4&feature=plpp_video[/video]