OK, from the beginning.
Phosphates should be almost undetectable but with RO only you may never see 0 phosphates if you have any amount to begin with. Phosphates are a tough thing to remove and require the combination of RO and a good DI to get to a true 0.
The TDS level from an RO only unit should be 95-98% less than what the tap water TDS was to begin with. Obviously the lower the final TDS the better, so stick with name brand RO membranes like a Dow Filmtec 75 GPD or maybe Applied Membranes or GE Water(Osmonics). There are no name Chinese knock offs on the market that do not carry ANSI/NSF ratings for sale and use in the USA for drinking water so pay attention to what you buy and always buy from a trusted source. Also stay away from the Dow 100 GPD membrane as it is NOT a reverse osmosis membrane but is a 90% efficient nano fiter ANSI/NSF rated for "Pool and Spa Use" and not drinking water which is much better.
A RO only TDS of 4, down from a tap water TDS of 160 is pretty good. You are getting a 96.25% rejection rate which is acceptable to most people. What you are referring to as "Charcoal" is actually activated carbon, yours may be granular activated carbon or it may be a solid carbon block which is much more efficient and has a much greater lifespan.
The poly prefilter and the carbons main duty are to protect the RO membrane with the prefilter removing suspended solids down to whatever its micron rating is (the smaller the better like less than 1.0) and the carbon removing chlorine. The carbon also does double duty by adsorbing volatile organic chemicals so it helps the membrane out. Important things to remember are, always replace the prefilter and carbon at 6 month intervals so you do not damage the membrane or pose the risk of bacteria or viruses growing inside the housings. Always disinfect the housings while you have the filters out, your vendor should have given you directions on how to do so and this is important! Next is always use a solid carbon block again in as low a micron rating as you can get, I prefer a 0.5 micron which is good for up to 20,000 gallons of normally chlorinated water. A cheap standard granular activated carbon cartridge can be exhausted in as little as 300 gallons, that's 300 gallons total through your RO system including the waste since it also goes through the carbon! That's like 60 gallons of good RO water and 240 gallons of waste with a normal 4:1 waste ratio. Not ver ygood at all when you think about it!
Now, the prefilter and carbon have very little to absolutely no effect on final TDS as I said they are there to protect the RO membrane which is what removes Total Dissolved Solids or TDS as we know it. If your TDS is rising then its the membrane that would need to be replaced but at 96+ % you are still in good shape with such low TDS to begin with. For me with a tap water TDS of 830 I would be looking for a new membrane at 96% because I would be getting an RO only TDS of 33 and be going through DI like crazy.
If you want to see 0 TDS which most of us do then you need a DI filter after your RO. Look for a full sized 10" vertical canister type with a refillable cartridge that holds 20 oz of resin for best treatment, best filter life and lowest replacement cost since you can buy bulk resin cheaper than throw away cartridges.
I will say Phosphates, Silicates and Nitrates are three of the harder things to remove as they are weakly ionized or electrically charged so even DI can have trouble. Two things you might consider, one is let the water travel through the DI resin bed very slowly so you have good contact time and the second is look into a DI cartridge designed specifically to work on phosphates and silicates like the SilicaBuster from
www.spectrapure.com .
You water utility is probably feeding orthophosphates for corrosion control in the Cities distribution system.
To sum it up, use high quality replacement filters in low micron ratings, a solid carbon block again in a low micron rating, add a DI and stick to a regular replacement/maintenance schedule and you should have good reef quality water for a long time.
