For the first six months of this year, my reef has been dealing with some strange losses that I haven't been able to positively pin on any one culprit. I've sent several water samples for ICP testing from two different companies, both of which agree my nitrates are much too high.  

But could it be anything else? One thing that hit me was how old my DI cartridge is.  It has been fully orange for a long time, but the TDS coming out of it continues to measure 0.  Granted, the TDS going into the resin is 2, so it should last a long time.  Today I wondered if maybe my TDS meter is giving me a faulty reading, that the water isn't coming out zero at all.  The odds that both probes are bad is highly unlikely, but looking at the resin there is no trace of the original dark color I'm used to seeing. 

Regardless of the numbers, I decided to just change out the cartridge. Additionally, the pre-filter set of filters were changed.

Will I now see some improvement in my reef? After all, I use RODI water for top off daily, so making sure the water going in is pure is and always has been my #1 priority.  Waiting for the TDS to start rising seems silly, when I can swap out everything so easily.