The man, the myth, the legend...thank you PaulB for commenting. I appreciate the insight. I will stay the course. I'm still retaining the option of changing out all of the rock. Though it's not what I'm planning right now, it will still be an option down the road.This is easy. When this hobby started we kept copper in the water continuously because we didn't know what we were doing and neither did anyone else. We actually didn't even have liquid copper so we used pennies.
Anyway, those rocks and gravel are still in my tank almost fifty years later but I did wait a few years to put in corals.
There were no corals for sale anyway.
But in your case, I would have removed those few rocks you have to a five gallon bucket with salt water. Then I would have gotten red, clay bricks like they sell in Home Depot for a buck or so each and used those because fish need hiding places and they are not afraid of bricks like they are of PVC pipes.
I use bricks all the time. After the fish are cured, I would change a good part of the water and add cuprisorb or carbon until there is no more copper. Then remove the bricks and maybe build a bar be cue out of them and replace the rock after you fresh water dip them for a few minutes.
Son't worry about copper being absorbed by the silicone as that is an old wives tale that any old wife can tell you.
After the fish are cured, try to go the natural route so they never get sick again.
Good Luck.
BTW, your Hippo tang also has HLLE
I did see the HLLE on the hippo. It's embarrassing but I'm doing what I can to combat it. My Yellow tang also has the same problem with HLLE but that prsisted from the last QT he went through a year and a half ago.