I am looking to buy an RO/DI unit but really only for water changes on my 20g. That being said, can anyone recommend a cheap RO/DI unit for me? I am not looking to spend a great deal of money but I do realize that these things aren't usually cheap. I have been afraid to buy one of the portable ones off ebay and wasn't sure if they worked that well at getting a near 0 TDS. Thanks in advance!