Looking at all of the posts, I noticed that you mention about ammonia at 25 ppm. Of all of the chemicals most toxic is ammonia. To get rid of it along with nitrite and nitrate, look into buying Instant Ocean Bio-Spira. It is bacteria and they advertise that adding, it will remove all NH3, NO3 and NO3 and fish can be added immediately.It depends on you a bit. Are you just using it for the tank or for drinking water too? I use mine for both, have the RO plumbed into my fridge ice maker, bypassing the DI portion unless I’m using it for the tank then I run it through the DI resin too. I bought mine from BRS. RO buddy is OK for small volumes but filters are pricy, it is available probably here at SWF..
You can buy on Amazon and also read all about the product
Hope you have success.