I don't think ceramics are porous enough to allow bacteria to live deep inside it in dangerous volumes. Overfilling the tank with warm water and a good dose of chlorine bleach should effectively wipe out the 'organics.' Though I think most people would make tea with tank water if the option was available.

Chemical contaminants are a bit trickier. It depends on what you've put in the tank; and that's difficult to know for certain. Ceramics are less vulnerable to leaching than, say, polyethylene, which is notorious for lasting contamination. Scrubbing with soap and warm water should reduce the risk, and the ongoing dilution from use should reduce levels over time, but there's no way to know the exact levels. If you were desperate enough to need that water to stay alive, I guess you would drink deep and take your chances.