Take down the whole tank dude. wash everything thoroughly. put the Clown in a separate tank, treat the clown until he has no more symptoms. i read on here somewhere it will take 8-10 weeks to starve out the ick
You had two tangs in a 30 biocube?
A huge part of ich comes from tangs that are stressed in their environment. That tank is way too small to keep one tang, two tangs would have been even worse.
Most tangs need 125 gallon and would be better in a 180.
Take the clown out and treat in a separate tank. Leave the tank without fish for 10 weeks so the ich dies.
When you restock, get fish that are ok in a small tank. You were way too overstocked.