you can use 50% RO and 50% tap mix. just use a water conditioner. I use pure RO to top off the tank due to evaporation.
I have a 40 breeder and I just use an aquaclear 70 hang on back filter. I have a sponge on the intake tube so no shrimp gets through.
If you HAVE an RO unit, then use it. if you need to buy one, dont.