I have 3 large pieces of mopani in my 75. When it was new, I started the soak.....changing the water every few days. I soaked for 6 weeks....was still getting tannins. Now, these pieces were too big to boil. I put them in the tank. Added 2 Purigen packets to each canister, they totally snatch up any hint of tannin. I change them every few weeks. They are always dark brown when I take them out.....so, to make it short, I say find a way to use Purigen.