"Tannins" (or Tannic Acid) will leach out of most wood... which is causing the brown coloration...
The hotter the water it is soaking in, the faster it will leach out... Thus boiling it is the fastest way to get it to leach out...
If it is to large to boil on the stove, then soaking it in a large enough container in as hot as possible/practical water is the next best step...
refreshing the water frequently also helps.
I've used a large (clean) rubbermaid garbage can to soak large wood using tap water as hot as my tap could provide... doing a 100% water change daily... This method worked quite well for me and was rather simple...
Each piece of wood will vary in the amount of time it will take to leach the vast majority of Tannic Acid out of...
Since your wood was collected in fresh water, after removing the tannins I suggest soaking it in very hot saltwater for a week or so. This will considerably reduce it's potential to rot in the tank. You can use aquarium salt or table sale for this step.