If you’re having issues with deionization resin exhausting too fast in your reverse osmosis filter, you might consider adding more stages before the RO membrane or even using a higher purity membrane. Because DI resin and membranes are quite pricey, it might be more economical in the long run to have to replace pre-filters, especially those inexpensive small micron sediment filters. If you really want to make the DI resin last, using a 99% rejection rate membrane will help too. And if all else fails, some DI resins can be recharged. It all costs money, no matter which path you take, but the point is finding the route that hits your pocketbook less.