There are several things that are key.
1. A high quality, low micron sediment filter. The purpose of the sediment filter is to protect the carbon blocks billions of tiny pores where the chlorine is adsorbed. 1 micron or smaller and absolute or near absolute rated are better than nominal rated.
2. A high quality, low micron carbon block 1 micron or smaller. This will adsorb the chlorine and if you have chloramines also the the chlorine portion of chloramines and break the bond with ammonia which is removed by the RO membrane and the DI. No special or multiple carbons are needed nor wanted.
3. The Dow Filmtec 75 GPD RO membrane is by far the most popular choice. Spectrapure and Buckeye Hydro also offer tested versions of this membrane so it performs better saving you money on DI replacements. Spectrapure even offers a hand tested 99% guaranteed membrane.
4. A full size 20 oz vertical DI filled with fresh resin, not something sitting on a shelf or boat shipping container somewhere. The little horizontal clear tubes do not compare since they lead to channeling and short circuiting of the water for poor treatment.
5. A capillary tube flow restrictor you the end user can trim so your waste ratio matches you exact water conditions not a preset restrictor that is usually ballpark close at best.
Three vendors I suggest in order are Spectrapure, Buckeye Hydro and PurelyH2o. All have been around a long time, 25 years for Spectrapure, 15 for Buckeye and Purely had its roots in Aquatic Reef Systems which is about the same age or older.
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BRS is not one of my recommended vendors for RO systems and supplies but love their chemicals and medias and use the heck out of them.