I am running carbon now.
I don't have a UV. If this was your tank would you consider buying a UV? Or ozone? ..
I was running Zeovit for a while, also ran Reef-resh before that. Not currently running either of those at the moment though. But I am using Taylor's "NO3 Destroyer".
I did in the past try a sulfur denitrator on this tank, but it never worked, so I gave up on that idea. That was nearly two years ago, would there be any residual sulfates? Even if there were, I thought they were supposed to be inert?
Throw some polyfilter into the mix maybe?? If it's something toxic I don't know what else I can do.
Basically all I can think of is things like Chemi-pure, copious carbon, and polyfilter. I guess I can try UV but I don't just have the understanding behind it to know what size and flowrate to look for.