I think to qualify as a dungeon, it must be damp and contain women in chains.
I think you'll wind up tearing it down and starting over. If I were faced with your situation, I would do one of these things:
1. Tear down and start completely over. Problem is, I don't know if there is something in the tank material (acrylic/glass) that is harboring poison. I saw a description about super-cleaning the tank; I would just buy another if I couldn't eliminate the impregnated poison possibility. I'm not saying that's what you should do, I'm saying that's one of the things I'd think of doing.
2. Give it a little more time, but while living through this probationary period, I'd start a completely separate, new 20g tank. Go through all the setup and satisfy myself that it is only the 75 that is sick. Gain a little experience and confidence.
3. Throw in the towel completely. This solution bugs me though because I'd never know what went wrong or how successful I could have been.
My two cents.