softener (get a di vessel and fill it with softner resin (not DI resin) then get a salt chamber (same as prefilters ask pure freedom they sell them, they sell the whole kit.
Softner does as it says and protects the membrane to a agree as it is already doing work on the water, the resin can be regenerated thats where the salt part comes in, granulated salt (you can get if from b&q) you fill the chamber with salt, disconnect from ro and slowly flush it through into the resin for about 20 min or till the salt if all gone into the resin (do not do it quick as it will do nothing) dont flush it through the ro either.
It is used for very hard water areas
it lowers the tds out of the membrane,