I seriously doubt you'd feel mercury poisoning after eating a single fish! Since you were already feeling ill chances are it's a garlic reaction. The bones will still be fine for broth even if there is a bit of mercury in the fish, as the mercury imbeds itself in the fatty tissues, not bones, whew! Good on you for making fish broth! I haven't done fish yet!
Mercury detoxing will happen naturally so long as you sweat, take in good minerals and fats and cleanse your liver and bowels. :) It's a very positive side effect to getting cleaner - metals chelation starts happening naturally! :) Chlorella, blue-green algea and spirulina help the process.