I finally got my new 4-stage RO/DI system set up last night, and all I can say is WOW! For the last year and a half I'd been using a cheap eBay unit... I'd never changed the membrane or the filters, and it wasn't that good to begin with. I also got a TDS memter with it. My old water tested around 20ppm, and my new
water tests at ZERO PPM!!! I'm hoping this will take care of a little algae problem in my tank and overall make the system more stable and healthier at the same time. And, since it's from an actual online company, I know exactly what replacement filters to get and I can change the DI as often as I need to. It was definitely worth the extra dollars to buy a real RO unit specifically made for aquarium use.
-Joe
p.s. If anyone wants the old five-stage unit I had, let me know. I'll take a couple sps frags or $20 in return. I'll bet you can buy new filters at a hardware store that'd work in the canisters. Then all you'd have to do is get a new DI filter for it if you wanted that option. The membrane is good, it's just that the DI and the prefilters should be changed out. If you did that the unit would start producing good, really pure water again. Or you could just use it as-is. 20ppm isn't all that bad, considering my source water is around 150-160ppm.
I used to get my water from Mega. From what I could tell it was only RO, and not that great RO water at that. I finally broke down and bought one as well. Gotta love it when the TDS reads zero 
Hope this unit gives you years of good service.
