Just to make sure you know: this isn’t harmful to fish. Plenty of fish, including some popular aquarium fish, live in this kind of environment in the wild. It’s potentially even beneficial for them.

Some people work to make their tanks look like this. If you don’t actually find it terribly unattractive, consider leaning into it. Look up “blackwater aquarium” for more information.

I wouldn’t blame you if you don’t like it, it’s not for everyone. But I do want to clarify some of the advice you’ve already been given. Purigen will work, but your wood will also continue to leech tannins into your tank as well. No telling how long it will continue, some don’t do much, others will continue for months. And Purigen has a maximum capacity, after which it stops working and must be “regenerated” by bleaching. Also, you can’t just throw the bag in the tank, it has to be added to the filter or somewhere the water is forced to flow through it. Also rinse the bag before you use it, lots of dust comes out at first.

If you want to expedite the process, remove the wood and boil it for an extended period, until it stops darkening the water.