Wow. Did you boil it first? I just finished boiling mine and plan to put it in tonight. Hope it's going to be alright. Mine is a new tank so I'll let you know if I got the same problem. Hope not.
I have 2 pieces of Mopani wood in a 110 gallon tank. I 'boiled' it, ran it in the shower under hot water for days, soaked and rinsed it for a few weeks more, and still over 2 years later it turns my water tea color. Actually it looks nice, and I think it brings out the colors in my fish. I wont get rid of the wood, and I really dont expect it to stop leaching any time soon.
It's tannins and will go away eventually with water changes and whenever it decides to stop leeching in to your water.You can take a rubbermaid tote,fill with water and let it leech there and do water changes until its done but no other way to avoid the leeching.Its gonna leech until its "done".
water changes help everything