I was told after the softener is better. That's also how my drinking water ro is plumed too.

 

This is just based off my intuition, but given it’s easier to use the softened water, and that the water after the softener has less stuff in it which would potentially prolong the life of your RO membrane and prefilter blocks, I want to say that is the play.

23 TDS seems like a lot though that is passing through the membrane. Perhaps your water is just that hard but could you also share what kind of RO system are you using for this?

 

My RO unit is after the water softener....the softener strips out Ca and Mg (and maybe some other elements) from our awful well water and provides us with household-ready water....the RO unit further refines it for tank use with very low TDS (it's 0 TDS when the DI resin is new)...I see the softener as sort of a pre-filter for the RO unit and it does a lot of the heavy lifting

 

As mentioned, after the softener is best in order to prolong life of membranes and filters.

 

Thank you everyone! It sounds like a consensus, to put it after the softener, which is good for me!

To @puopg about my current kitchen RO unit outputting 23 PPM TDS, here's a thread where I investigated some issues with my water upon receipt of some testing strips and a hand held electronic analyzer. Note that the water (softened) starts at around 700 TDS so I can't complain too much! The RO is taking out more than 29 out of every 30 dissolved impurities! In this thread I have a link to the one I'm using. The water from it tastes fantastic.

Considering my water strategy for planted...

Hi folks, My 90gal tank remains dry so far but with help from this forum I've got a lighting strategy planned (Chihiros RGB2) and while I await the spring to get the tank going (for various reasons, setting up now isn't practical), I'm putting things into place. This will be my first tank in a...

www.plantedtank.net

Note that I've since found with my hand held analyzer that bottled DI water near me (measured electronically) has TDS around 6-10 PPM varying from jug to jug.

I will be getting a separate unit for tank purposes. Much planning to do!

 

Note that the water (softened) starts at around 700 TDS

You truly do have liquid rock

 

Seriously, I considered starting an African cichlid tank instead, but I really want loaches and angelfish.

I'm planning on getting the same APEC brand RO-90 unit for my fish work as I have in the kitchen. That way I can buy filters in bulk and they are interchangeable.