I'm in the same boat. I was buying 5gallon boxes from petco but it was 11.99 plus tax for each box. When I first set up my tank I didn't wanna spend that money so luckily my friend let me use his rodi
machine. I have a. 55 gallon. I ended up buying my own off eBay for like 75$ shipped but it was used. The Tds
reader was showing over 100ppm. I just realized this last week and been dojng top offs and water changes since November
I changed all the filters except the ro
membrane still read 100 it went down like 40 ppm
. I hope this solves my issue. Basically where I'm getting at is the most affordable is to but a brand new rodi
unit. Brs has em for like $130. It's a lot up front but totally worth it in the end.