No quick fixes that I know of I'm afraid, I had 1 piece that stopped leeching after about a week and a couple of pieces of mopani that seemed like they'd never stop no matter how long I soaked them, in the end I bit the bullet and placed them in to a tank and they seem to have been okay since. I do have a large piece on standby for the new tank when it comes and I have no idea yet as to how long that's going to take. I think its either a case of living with the tannins until they stop leeching naturally or trying something like purigen in the filter.