My DI turns brown after my second 30 gallon water change but still maintains 3 TDS or under on the output for about 3 months. I personally wait until it reaches 5 TDS before changing it.
After turning brown what is your TDS reading?
High co2 will kill resin life. If you have well water then thats prob the problem as that was mine. Easy fix is to run the water out of the membrane to a container with a airstone then pump it back into your di resin chamber. If you need more info just send.me a PM