Assuming GAC is removing the smell, the yellowing, the organics, etc, taking this "dirty" water out of the tank and then replacing it with freshly made saltwater is pretty much the same thing I guess. That bucket of freshly made saltwater is as pure as it gets though. I know what I'm getting through water changes. No worries about what it might leach, what "good" stuff it might remove along with the bad, no HLLE speculation, nothing. To be honest with you, I really don't think anybody knows for certain how long carbon lasts. Just as an example, let's say you replace your carbon every 2 weeks. Well, after five days in the smelly yellow tank water it has exhausted itself, it's done. If you weren't seeing anything out of the ordinary when it was time to change it out 14 days later, you have pretty much been going without carbon for the last 9 days. This type of thing happens all the time. Lets face it, GAC could fall off the face of the earth and we still get by. Water changes, no so much.
Happy reefing.
