Try removing the DI resin canister and check what the RO membrane is putting out. Old DI resin is just as bad. I had bag of resin that tore open and dried out. The TDS out of the resin was higher than what was going in.
If you can get the TDS value of your tap water, you can calculate the rejection ratio of the membrane. 1-(output/input) ie, 1-(3/100) = 1-0.03 = 97% anything below 95 should be tossed.
Bulk reef supply and buckeye hydro have good prices on DI resin. RO membranes I get on ebay. What size system is this? 75 gpd?