I've never used mopani driftwood before, but from experience, I found that it's almost impossible to stop the tannings completely. I've had two pieces of malaysian driftwood in my 20 gal for a year after hours of boiling and they still leech tannins so I have to do water changes every 3 days to keep it looking nice. I just ordered 15 pounds of seiryu stones so this should get rid of my problem! My girlfriend also got a piece of malaysian driftwood for her 5 gal. I boiled it for probably a total of 8 hours and it still leeches within two or three days! The only solution I have for you is to do water changes every few days. Sorry if I didn't help that much..