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If you are using a TDS or conductivity meter to monitor the salinity of a marine aquarium, you will have to be able to read up to about 53 mS/cm (about 53,000 ppm of 442 equivalents). I do not suggest trying to determine salinity from diluted samples, as the conductivity of seawater does not drop linearly with dilution.
90% rejection is not good. Most membranes including the most popular Dow Filmtec 75 GPD as well as the GE and Applied membranes are rated at 98% rejection and should be removing 96% at a minimum.
The difference between 90 and 98 % is very dramatic and can be a big cost savings when it comes to DI. A very accurate rule of thumb states "For every 2% you increase the RO rejection rate, you DOUBLE the life of your DI resin". This means increasing the rejection to 98% will make your DI last what, maybe 4 to 8 times longer than at 90%.
fsu1dolfan, a TDS of 2 from RO only is pretty good but you still have no idea what that 2 TDS is made up of. Certainly better than tap but it can be improved upon for maybe $30 with an add on DI filter kit. The reason its important to have 0 TDS is some contaminants are very weakly ionized and may not register on a TDS meter when present. You could have higher levels of phosphates or silicates and not know it as only some may register. Another drawback to RO only is RO is poor at removing all forms of ammonia including nitrites and nitrates. Some Utilities (roughly 30% in the US) are now using chloramines as a residual disinfectant and the RO will pass the ammonia. DI is what removes the ammonia portion of the chloramines while good carbon gets the chlorine portion.
this may be a little OT btu if my tap water tests under 10 TDS is it even worth it to look into gettign RO/DI