RO/DI question

[ensquire]

Finally picked up a TDS meter and the water is pretty good. 26ppm.
I have been running my 36 gal with treated tap water for 2 years. Mostly softies and LPS with the fish and things have been fine. I just bought a used spectapure 100 GPD RO/DI setup. Do I need all cartridges to deal with such low TDS. Which membranes should I use. Being that low, I am tempted to just use the treated tap water. Only because I don't really have a place to set it up at my home. Opinions please.