Are you sure it was ich?
I concur with the poster who suggested not adding any fish until several weeks after any illness or symptoms have subsided.
I asked about ich because it may have been something else. Brooklynella is quite common in Royal Grammas (although usually more so during the hotter months). If Brooklynella was in the tank, and you added a clown, Brook can take a clown in as little as a few hours (although they don't always die from it.)
Cardinals in general rarely get sick (with the exception of the questionable pathogen that wild Banggais seem to suffer from, but that's another post). It's quite possible that the Cardinal is either not prone to, immune to or a carrier of whatever got your Gramma and Clown.
What are your parameters? Please post specifics. I don't think the "additives" had anything to do with it - the resins (SeaGel and Purigen) are fine - Matrix isn't really necessary if you have live rock (or was it Matrix Carbon? If so, that's fine). The trace elements and buffers didn't likely have anything to do with it either.
Don't sweat the bristleworm - they are good for your tank. I'm sure you probably have more of them (hiding) - and that's a good thing. They don't harm healthy tissue (fish, coral or inverts), but they will clean up the dead or dying. They are important janitors, so don't try to remove them - if you have an unusually large population of them it could be due to overfeeding - but you did not indicate that was an issue in your initial post.
Hope this helps.
Jenn